mtd: mtdconcat: Check _read, _write callbacks existence before assignment

Since 2431c4f5 ("mtd: Implement mtd_{read,write}() as wrappers
around mtd_{read,write}_oob()") don't allow _write|_read and
_write_oob|_read_oob existing at the same time, we should check the
existence of callbacks "_read and _write" from subdev's master device
(We can trust master device since it has been registered) before
assigning, otherwise following warning occurs while making
concatenated device:

  WARNING: CPU: 2 PID: 6728 at drivers/mtd/mtdcore.c:595
  add_mtd_device+0x7f/0x7b0

Fixes: 2431c4f5 ("mtd: Implement mtd_{read,write}() around ...")
Signed-off-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210817114857.2784825-3-chengzhihao1@huawei.com

drivers/mtd/mtdconcat.c

@@ -694,6 +694,10 @@ struct mtd_info *mtd_concat_create(struct mtd_info *subdev[],	/* subdevices to c
 		concat->mtd._block_markbad = concat_block_markbad;
 	if (subdev_master->_panic_write)
 		concat->mtd._panic_write = concat_panic_write;
+	if (subdev_master->_read)
+		concat->mtd._read = concat_read;
+	if (subdev_master->_write)
+		concat->mtd._write = concat_write;
 
 	concat->mtd.ecc_stats.badblocks = subdev[0]->ecc_stats.badblocks;
 
@@ -755,8 +759,6 @@ struct mtd_info *mtd_concat_create(struct mtd_info *subdev[],	/* subdevices to c
 	concat->mtd.name = name;
 
 	concat->mtd._erase = concat_erase;
-	concat->mtd._read = concat_read;
-	concat->mtd._write = concat_write;
 	concat->mtd._sync = concat_sync;
 	concat->mtd._lock = concat_lock;
 	concat->mtd._unlock = concat_unlock;