mmc: sdhci-acpi.c: Use SET_RUNTIME_PM_OPS macro to set runtime pm callbacks

This allows us to get rid of the #else condition, as the macro compiles
away to nothing if not enabled.

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