mmc: core: always check the length of sglist with total data size

All the check within mmc_mrq_prep seems to be all-or-none
proposition, so it doesn't make sense to only check the
length of sglist only under the CONFIG_MMC_DEBUG context.
I'd prefer to always keep the check there unconditionally.

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