mmc: fix the build error when MMC_WRITE is disabled

erase_grp_size is used, when MMC_WRITE is enabled.
- error: ‘struct mmc’ has no member named ‘erase_grp_size’

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>