We've found that while in host mode, using Android, if one runs
the command:
  stop adbd

The existing usb devices being utilized in host mode are disconnected.
This is most visible with usb networking devices.

This seems to be due to adbd closing the file:
  /dev/usb-ffs/adb/ep0
Which calls ffs_ep0_release() and the following backtrace:

[<ffffff800875a430>] dwc2_hsotg_ep_disable+0x148/0x150
[<ffffff800875a498>] dwc2_hsotg_udc_stop+0x60/0x110
[<ffffff8008787950>] usb_gadget_remove_driver+0x58/0x78
[<ffffff80087879e4>] usb_gadget_unregister_driver+0x74/0xe8
[<ffffff80087850c0>] unregister_gadget+0x28/0x58
[<ffffff800878511c>] unregister_gadget_item+0x2c/0x40
[<ffffff8008790ea8>] ffs_data_clear+0xe8/0xf8
[<ffffff8008790ed8>] ffs_data_reset+0x20/0x58
[<ffffff8008793218>] ffs_data_closed+0x98/0xe8
[<ffffff80087932d8>] ffs_ep0_release+0x20/0x30

Then when dwc2_hsotg_ep_disable() is called, we call
kill_all_requests() which causes a bunch of the following
messages:

dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
dwc2 f72c0000.usb: Mode Mismatch Interrupt: currently in Host mode
init: Service 'adbd' (pid 1915) killed by signal 9
init: Sending signal 9 to service 'adbd' (pid 1915) process group...
init: Successfully killed process cgroup uid 0 pid 1915 in 0ms
init: processing action (init.svc.adbd=stopped) from (/init.usb.configfs.rc:15)
dwc2 f72c0000.usb: dwc2_hc_chhltd_intr_dma: Channel 8 - ChHltd set, but reason is unknown
dwc2 f72c0000.usb: hcint 0x00000002, intsts 0x04200029
dwc2 f72c0000.usb: dwc2_hc_chhltd_intr_dma: Channel 12 - ChHltd set, but reason is unknown
dwc2 f72c0000.usb: hcint 0x00000002, intsts 0x04200029
dwc2 f72c0000.usb: dwc2_hc_chhltd_intr_dma: Channel 15 - ChHltd set, but reason is unknown
dwc2 f72c0000.usb: hcint 0x00000002, intsts 0x04200029
dwc2 f72c0000.usb: dwc2_hc_chhltd_intr_dma: Channel 3 - ChHltd set, but reason is unknown
dwc2 f72c0000.usb: hcint 0x00000002, intsts 0x04200029
dwc2 f72c0000.usb: dwc2_hc_chhltd_intr_dma: Channel 4 - ChHltd set, but reason is unknown
dwc2 f72c0000.usb: hcint 0x00000002, intsts 0x04200029
dwc2 f72c0000.usb: dwc2_update_urb_state_abn(): trimming xfer length

And the usb devices connected are basically hung at this point.

It seems like if we're in host mode, we probably shouldn't run
the dwc2_hostg_ep_disable logic, so this patch returns an error
in that case.

With this patch (along with the previous patch in this set), we avoid
the mismatched interrupts and connected usb devices continue to function.

I'm not sure if some other solution would be better here, but this seems
to work, so I wanted to send it out for input on what the right approach
should be.

Cc: Wei Xu <xuwei5@hisilicon.com>
Cc: Guodong Xu <guodong.xu@linaro.org>
Cc: Amit Pundir <amit.pundir@linaro.org>
Cc: YongQin Liu <yongqin.liu@linaro.org>
Cc: John Youn <johnyoun@synopsys.com>
Cc: Minas Harutyunyan <Minas.Harutyunyan@synopsys.com>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Chen Yu <chenyu56@huawei.com>
Cc: Felipe Balbi <felipe.balbi@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-usb@vger.kernel.org
Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Tested-by: Minas Harutyunyan <hminas@synopsys.com>
Reported-by: YongQin Liu <yongqin.liu@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>