mmc: block: Refactor mmc_blk_part_switch()

Instead of passing a struct mmc_blk_data * to mmc_blk_part_switch()
let's pass the actual partition type we want to switch to. This
is necessary in order not to have a block device with a backing
mmc_blk_data and request queue and all for every hardware partition,
such as RPMB.

Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>