mmc: Fix mmc bus width

mmc: Fix mmc bus width

After setting the bus width, the extended CSD register is read. Some selected
fields are compared with previously read extended CSD register fields. In this
comparison the EXT_CSD_ERASE_GROUP_DEF field is compared. But this field is
previously written under certain circumstances. And then the comparison fails.

Only compare read-only fields. Therefore compare field EXT_CSD_HC_WP_GRP_SIZE
instead of field EXT_CSD_ERASE_GROUP_DEF.

Signed-off-by: Mario Schuknecht <mario.schuknecht@dresearch-fe.de>
Acked-by: Pantelis Antoniou <panto@antoniou-consulting.com>