The MMC hosts could be left in an unconsistent or uninitialized state from
the firmware. Instead of assuming, the firmware did the right things, let's
reset the host controllers.

This change fixes a bug when the mmc2/sdio is initialized leading to a hung
task:

[  242.704294] INFO: task kworker/7:1:675 blocked for more than 120 seconds.
[  242.711129]       Not tainted 4.9.0-rc8-00017-gcf0251f #3
[  242.716571] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  242.724435] kworker/7:1     D    0   675      2 0x00000000
[  242.729973] Workqueue: events_freezable mmc_rescan
[  242.734796] Call trace:
[  242.737269] [<ffff00000808611c>] __switch_to+0xa8/0xb4
[  242.742437] [<ffff000008d07c04>] __schedule+0x1c0/0x67c
[  242.747689] [<ffff000008d08254>] schedule+0x40/0xa0
[  242.752594] [<ffff000008d0b284>] schedule_timeout+0x1c4/0x35c
[  242.758366] [<ffff000008d08e38>] wait_for_common+0xd0/0x15c
[  242.763964] [<ffff000008d09008>] wait_for_completion+0x28/0x34
[  242.769825] [<ffff000008a1a9f4>] mmc_wait_for_req_done+0x40/0x124
[  242.775949] [<ffff000008a1ab98>] mmc_wait_for_req+0xc0/0xf8
[  242.781549] [<ffff000008a1ac3c>] mmc_wait_for_cmd+0x6c/0x84
[  242.787149] [<ffff000008a26610>] mmc_io_rw_direct_host+0x9c/0x114
[  242.793270] [<ffff000008a26aa0>] sdio_reset+0x34/0x7c
[  242.798347] [<ffff000008a1d46c>] mmc_rescan+0x2fc/0x360

[ ... ]

Cc: stable@vger.kernel.org
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>