staging: most: hdm-usb: fix race between enqueue and most_stop_enqueue

The "broken in pipe" handler of the USB-HDM calls most_stop_enqueue() to
stop the MBO traffic before returning all MBOs back to the Mostcore.  As
the enqueue() call from the Mostcore may run in parallel with the
most_stop_enqueue(), the HDM may run into the inconsistent state and
crash the kernel.

This patch synchronizes enqueue(), most_stop_enqueue() and
most_resume_enqueue() with a mutex, hence avoiding the race condition.

Signed-off-by: Andrey Shvetsov <andrey.shvetsov@k2l.de>
Signed-off-by: Christian Gromm <christian.gromm@microchip.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>