mmc: core: Use mmc_flush_cache() during mmc suspend

Earlier we disabled the cache during suspend, which meant a flush was
internally at the eMMC performed as well.

To simplify code we can make use of the mmc_flush_cache(), during mmc
suspend, which makes the mmc_cache_ctrl() redundant so then we can
remove it.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Seungwon Jeon <tgih.jun@samsung.com>
Signed-off-by: Chris Ball <chris@printf.net>