mmc: core: Make mmc_switch_status() available for mmc core

Following changes needs mmc_switch_status() to be available both from mmc.c
and mmc_ops.c. Allow that by moving its implementation to mmc_ops.c and
make it available via mmc_ops.h.

Moving mmc_switch_status() to mmc_ops.c, also enables us to turn
mmc_switch_status_error() into static function. So let's take the
opportunity to change this as well.

Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Tested-by: Jaehoon Chung <jh80.chung@samsung.com>