mmc: fix erase_grp_size computation with high-capacity size definition

The erase_grp_size in struct mmc is to be a size in 512-byte sectors
but the code used to compute it for eMMC when EXT_CSD_ERASE_GROUP_DEF is
enabled computed it as bytes, leading to erase sizes and alignment
much larger than what is actually required by the mmc device.

Signed-off-by: Diego Santa Cruz <Diego.SantaCruz@spinetix.com>