mmc: cmdq_hci: Add a memory barrier before ringing doorbell

Ensure the task descriptor list memory is flushed before ringing
the doorbell by adding a memory barrier. Also commit the doorbell
write immediately to help improve performance.

Change-Id: I4f26f6da871858e42bf74ecb4a8cdd9e3375ee80
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>
Signed-off-by: Ram Prakash Gupta <rampraka@codeaurora.org>