mtdcore: Fix a build error with CONFIG_CMD_MTDPARTS_SPREAD

This patch fixes the build error for CONFIG_CMD_MTDPARTS_SPREAD
Signed-off-by: NMaxin B. John <maxin.john@enea.com>

drivers/mtd/mtdcore.c

@@ -803,7 +803,7 @@ void mtd_get_len_incl_bad(struct mtd_info *mtd, uint64_t offset,
 	*truncated = 0;
 	*len_incl_bad = 0;
 
-	if (!mtd->block_isbad) {
+	if (!mtd->_block_isbad) {
 		*len_incl_bad = length;
 		return;
 	}
@@ -819,7 +819,7 @@ void mtd_get_len_incl_bad(struct mtd_info *mtd, uint64_t offset,
 
 		block_len = mtd->erasesize - (offset & (mtd->erasesize - 1));
 
-		if (!mtd->block_isbad(mtd, offset & ~(mtd->erasesize - 1)))
+		if (!mtd->_block_isbad(mtd, offset & ~(mtd->erasesize - 1)))
 			len_excl_bad += block_len;
 
 		*len_incl_bad += block_len;