mmc: queue: fix for system suspend flow for eMMC command queue (cmdq)

System suspend executed on kernel PM core context. When there are active
requests system suspend is rejected. Semaphore used to resolve race
between mmc_cmdq_thread() and mmc_queue_suspend().

Change-Id: I88f668d7a6dd6403407ac8208265e4439b35173c
Signed-off-by: Konstantin Dorfman <kdorfman@codeaurora.org>