mmc: block: Move duplicate check

mmc_blk_ioctl() calls either mmc_blk_ioctl_cmd() or
mmc_blk_ioctl_multi_cmd() and each of these make the same
check. Factor it into a new helper function, call it on
both branches of the switch() statement and save a chunk
of duplicate code.

Cc: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>