mwifiex: do not return success when command times out

wait_event_interruptible_timeout() returns 0 upon timeout. We should
convert it to a negative error code (such as -ETIMEDOUT) instead of
returning it directly, as return code of 0 indicates that command was
executed.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>