mmc: Allow 2 byte requests in 4-bit mode for tmio_mmc

Adjust the tmio_mmc block size check to accept 2-byte requests in 4-bit
mode if the hardware supports it.

Tested with the SDHI hardware block included in sh7724.

Signed-off-by: Yusuke Goda <yusuke.goda.sx@renesas.com>
Signed-off-by: Matt Fleming <matt@console-pimps.org>
Acked-by: Magnus Damm <damm@opensource.se>
Tested-by: Arnd Hannemann <arnd@arndnet.de>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>