mmc: cqhci: Commit descriptors before setting the doorbell

Add a write memory barrier to make sure that descriptors are actually
written to memory, before ringing the doorbell.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>