mmc: block: Fix invalid data from freed request pointer

When we do blk_end_request, request pointer may get freed.
Therefore cache request_queue pointer and tag value at start itself
instead of dereferencing already freed request_queue pointer.

Change-Id: I35def3ef6a260ebe78e92874ac121aca529f00cf
Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>