mmc: dw_mmc: simplify optional reset handling

mmc: dw_mmc: simplify optional reset handling

As of commit bb475230b8e5 ("reset: make optional functions really
optional"), the reset framework API calls use NULL pointers to describe
optional, non-present reset controls.

This allows to return errors from devm_reset_control_get_optional and to
call reset_control_(de)assert unconditionally.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Link: https://lore.kernel.org/r/20210305090724.18832-1-p.zabel@pengutronix.de
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>