mmc: core: Use MMC_UNSAFE_RESUME as default behavior

Invoking system suspend or shutdown without using the Kconfig option
MMC_UNSAFE_RESUME, did trigger an ungraceful power cut of the card.

To improve the situation, change the behavior to always make use of the
available bus_ops callbacks that handles system suspend and shutdown
properly.

By changing the behavior MMC_UNSAFE_RESUME becomes redundant, so lets's
remove it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Signed-off-by: Chris Ball <chris@printf.net>