scsi: sync cache on write

scsi: sync cache on write

We don't have a mechanism to safely shutdown block devices prior to a
baord reset or driver removal. Prevent data loss by synchronizing the
SCSI cache after every write.

In particular this solves the issue of capsule updates looping on some
devices because the board resets immediately after deleting the capsule
file and this write wouldn't be flushed in time.

This may impact NAND wear, but should be negligible given the usecases
for disk write in U-Boot.

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>