mmc: sdhci-pci-core: Simplify code by using SET_SYSTEM_SLEEP_PM_OPS

Convert to define the system PM callbacks to be build for CONFIG_PM_SLEEP
and use the SET_SYSTEM_SLEEP_PM_OPS. In this way the code becomes cleaner.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>