mmc: sh_mmcif: Make sure the device stays active when needed in ->probe()

While accessing the device, make sure it stays active by increasing the
runtime PM usage count for it.

Let's also defer to enable runtime PM until we really need access to the
device. This also enables the error path in ->probe() to become simpler.

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