mmc: block: workaround for timeout issue with some vendor devices

Commit 66a7393a ("mmc: block: ensure CMDQ
is empty before queuing cache flush") added a workaround for particular
vendor's eMMC devices. Workaround was to wait for all the outstanding
requests to finish up before queuing the flush request. Now detailed
root cause analysis from vendor shows that original issue can happen only
if DCMD command is sent to device too quickly (within less than 6
microseconds) after completion of previous small sector (less than 8
sectors) read operations. Hence with this change, we are fine tuning the
previous workaround such that it would almost have no impact on the storage
benchmark performance numbers.

Change-Id: I1df1c5d7bbcd7b526236651077b7dade2626cb30
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>