ahci: Perform SATA flush after disk write.

Writes in u-boot are so rare, and the logic to know when is
the last write and do a flush only there is sufficiently
difficult. Just do a flush after every write. This incurs,
usually, one extra flush when the rare writes do happen.

Signed-off-by: Marc Jones <marc.jones@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>