Fix memory leak in mmc_read()

There is be a path through mmc_read in drivers/mmc/mmc.c where
malloc'd memory is not freed before exiting mmc_read: it occurs if
mmc_set_blocklen() returns a non-zero value.

Reported-by: Quentin Armitage <Quentin@Armitage.org.uk>
Signed-off-by: Wolfgang Denk <wd@denx.de>