mvsdio: fix handling of partial word at the end of PIO transfer

Standard data flow for MMC/SD/SDIO cards requires that the mvsdio
controller be set for big endian operation.  This is causing problems
with buffers which length is not a multiple of 4 bytes as the last
partial word doesn't get shifted all the way and stored properly in
memory.  Let's compensate for this.

Signed-off-by: Nicolas Pitre <nico@marvell.com>
CC: stable@kernel.org
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>