mmc: core: Convert mmc_driver to device_driver

The struct mmc_driver adds an extra layer on top of the struct
device_driver. That would be fine, if there were a good reason, but
that's not the case.

Let's simplify code by converting to the common struct device_driver
instead and thus also removing superfluous overhead.

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