mmc: Fix erase_grp_size for partitioned card

EXT_CSD_ERASE_GROUP_DEF is lost every time after a reset or
 power off. Set it if device has enhanced partitions.

Signed-off-by: Oliver Metz <oliver@freetz.org>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>