mmc: tmio: only access registers above 0xff, if available

Not all tmio implementations have registers above oxff. Accessing
them on thise platforms is dangerous. In some cases it leads to
address wrapping to addresses below 0x100, which corrupts random
unrelated registers.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Acked-by: Paul Mundt <lethal@linux-sh.org>
Signed-off-by: Chris Ball <cjb@laptop.org>