mmc: block: return error on failed mmc_blk_get()

This used to return -EFAULT, but the function above returns -EINVAL on
the same condition so let's stick to that.

The removal of error return on this path was introduced with b093410c
('mmc: block: copy resp[] data on err for MMC_IOC_MULTI_CMD').

Fixes: b093410c

 ('mmc: block: copy resp[] data on err for MMC_IOC_MULTI_CMD').
Signed-off-by: Olof Johansson <olof@lixom.net>
Cc: Grant Grundler <grundler@google.com>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>