Commit 77b0a099 ("cdc-ncm: use common parser") added a dangerous
new trust in the CDC functional descriptors presented by the device,
unconditionally assuming that any device handled by the driver has
a CDC Union descriptor.

This descriptor is required by the NCM and MBIM specs, but crashing
on non-compliant devices is still unacceptable. Not only will that
allow malicious devices to crash the kernel, but in this case it is
also well known that there are non-compliant real devices on the
market - as shown by the comment accompanying the IAD workaround
in the same function.

The Sierra Wireless EM7305 is an example of such device, having
a CDC header and a CDC MBIM descriptor but no CDC Union:

    Interface Descriptor:
      bLength                 9
      bDescriptorType         4
      bInterfaceNumber       12
      bAlternateSetting       0
      bNumEndpoints           1
      bInterfaceClass         2 Communications
      bInterfaceSubClass     14
      bInterfaceProtocol      0
      iInterface              0
      CDC Header:
        bcdCDC               1.10
      CDC MBIM:
        bcdMBIMVersion       1.00
        wMaxControlMessage   4096
        bNumberFilters       16
        bMaxFilterSize       128
        wMaxSegmentSize      4064
        bmNetworkCapabilities 0x20
          8-byte ntb input size
      Endpoint Descriptor:
	..

The conversion to a common parser also left the local cdc_union
variable untouched.  This caused the IAD workaround code to be applied
to all devices with an IAD descriptor, which was never intended.  Finish
the conversion by testing for hdr.usb_cdc_union_desc instead.

Cc: Oliver Neukum <oneukum@suse.com>
Fixes: 77b0a099 ("cdc-ncm: use common parser")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>