mmc: core: Fix null pointer dereference due to race conditions

Fix race condition between mmcqd thread and the mmc_queue_suspend
updating a shared variable mq->flags, which can lead to potential
null pointer dereference as following-

Unable to handle kernel NULL pointer dereference at
virtual address 00000020
pgd = c0004000
[00000020] *pgd=00000000
mmcqd/0:  186] Internal error: Oops: 5 [#1] PREEMPT SMP ARM
CPU: 0    Tainted: G        W     (3.4.0-1251694-eng #1)
PC is at mmc_blk_err_check+0x20c/0x3b8
LR is at mmc_start_req+0x198/0x718

cpu0		|	cpu1
x |= 1		|	x |= 2

final value of x can be x = 1 or x = 2

Change-Id: Ie0fff6d6dba5aebb3584cba9fb98de24515c4cd8
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>