mmc: core: Fix null pointer dereference due to illegal mmc request

Fix a race condition that can lead to null pointer dereference
while the MMC transfers are going on.

1) mmc_request_done() -> mmc_wait_for_data_done ->
	step1: update is_done_rcv
	step2: wake_up sleeping thread (mmcqd) waiting for is_done_rcv

2) mmcqd -> mmc_wait_for_data_req_done
	step4: wait for is_done_rcv or is_new_req
	step5: is_new_req set from block layer context and mmcqd
	       is woken up
	step6: let's say step1 is done, so complete the current request
	step7: fetch new request and issue to host layer
	step8: fetch one more request and wait for previous request to
	       complete

In the above execution contexts, if step4-step8 happens between step1 and
step2 a null pointer dereference is observed -

[   29.483302] Unable to handle kernel NULL pointer dereference
		at virtual address 00000488
[   29.490366] pgd = c0004000
[   29.493054] [00000488] *pgd=00000000
[   29.518937] PC is at do_raw_spin_lock+0x8/0x13c
[   29.523445] LR is at _raw_spin_lock_irqsave+0x20/0x28
[   30.108789] [<c0339cd4>] (do_raw_spin_lock+0x8/0x13c) from
[   30.118418] [<c095d178>] (_raw_spin_lock_irqsave+0x20/0x28) from
[   30.127445] [<c0142ef4>] (__wake_up+0x20/0x50) from
[   30.136124] [<c0663f70>] (mmc_request_done+0x30c/0x368) from
[   30.145932] [<c067bd98>] (sdhci_tasklet_finish+0x130/0x13c) from

Change-Id: I9a21431b5fd9bb9bbcb5c18a9895096fe845e64b
Signed-off-by: Sujit Reddy Thumma <sthumma@codeaurora.org>