mmc: tmio_mmc: silence compiler warnings

with "mmc: tmio: implement a bounce buffer for unaligned DMA"
gcc generates the following warnings:

drivers/mmc/host/tmio_mmc.c:654:6: warning: 'ret' may be used uninitialized in this function
drivers/mmc/host/tmio_mmc.c:730:6: warning: 'ret' may be used uninitialized in this function

This patch fixes these by setting ret to -EINVAL in the affected code paths.

Signed-off-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: Chris Ball <cjb@laptop.org>