MMC: unify mmc read and write operation

mmc read and write command has so many in common, unfiy those two to
force consistency across the those two.

Signed-off-by: Lei Wen <leiwen@marvell.com>
Acked-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Andy Fleming <afleming@freescale.com>