mmc: block: ensure CMDQ is empty before queuing cache flush

Some devices might stop responding to new commands if device cache flush
is queued to host controller while host controller is still processing
outstanding requests.

To workaround this issue, we are making the cmdq thread to wait for all
the oustanding requests to be finished before queuing cache flush command
on host controller.

Change-Id: I15387734f51ca4cadfc9e11270f14d8a0806a00f
Signed-off-by: Subhash Jadavani <subhashj@codeaurora.org>