mmc: sh_mmcif: Add exclusion between cmd and interrupt

A command end interrupt should not be processed between command issue
and setting of wait_for flag. It expects already the flag to be set.
Therefore the exclusive control was added.

Signed-off-by: Kouichi Tomita <kouichi.tomita.yn@renesas.com>
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>