mmc: matsushita-common: Correctly set mode in 16bit

The HOST_MODE register must be set to 0 when the IP is operated in 16bit
mode, otherwise 16bit access to the data FIFO may fail.

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