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: I321d5bed95b802d4bcc00836ce9cdede316b29f5
Signed-off-by: Venkat Gopalakrishnan <venkatg@codeaurora.org>