mmc: matsushita-common: Special case only select registers in 16bit

mmc: matsushita-common: Special case only select registers in 16bit

There are only a few registerse used in the 16bit mode which are
32bit internally. Special-case only those in the IO accessors and
always write both halves. Any other register access is protected
from accidentally overwriting neighboring register.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Jaehoon Chung <jh80.chung@samsung.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>