mmc: dw_mmc-pltfm: Remove superflous #else condition on CONFIG_PM_SLEEP

As the code is using SIMPLE_DEV_PM_OPS helper, this compiles away to
nothing if CONFIG_PM_SLEEP is disabled. Thus we don't need to #define
the suspend/resume callbacks to NULL.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>