mmc: block: Reparametrize mmc_blk_ioctl_[multi]_cmd()

Instead of passing a block device to
mmc_blk_ioctl[_multi]_cmd(), let's pass struct mmc_blk_data()
so we operate ioctl()s on the MMC block device representation
rather than the vanilla block device.

This saves a little duplicated code and makes it possible to
issue ioctl()s not targeted for a specific block device but
rather for a specific partition/area.

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