mmc: at91_mci: update bytes_xfered value once xfer done

Modify bytes_xfered value after a write.

That will report, as accurately as possible, the amount of
sectors that are effectively written.

This update introduces the check of the busy signal given by
the card.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Pierre Ossman <drzeus@drzeus.cx>