mmc: core: Move mmc_flush_cache() from core.c to mmc_ops.c

The mmc_flush_cache() is a eMMC specific function, let's move it to
mmc_ops.c to make that clear.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Shawn Lin <shawn.lin@rock-chips.com>