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: NJaehoon Chung <jh80.chung@samsung.com>
Reviewed-by: NPeng Fan <peng.fan@nxp.com>

drivers/mmc/mmc.c

@@ -1126,9 +1126,11 @@ int mmc_hwpart_config(struct mmc *mmc,
 
 		ext_csd[EXT_CSD_ERASE_GROUP_DEF] = 1;
 
+#if CONFIG_IS_ENABLED(MMC_WRITE)
 		/* update erase group size to be high-capacity */
 		mmc->erase_grp_size =
 			ext_csd[EXT_CSD_HC_ERASE_GRP_SIZE] * 1024;
+#endif
 
 	}