UBI: block: Fix checking for NULL instead of IS_ERR()

We recently switched from allocating ->rq using blk_init_queue() to
use blk_mq_init_queue() so we need to update the error handling to
check for IS_ERR() instead of NULL.

Fixes: ff1f48ee3b ('UBI: Block: Add blk-mq support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/block.c

@@ -409,9 +409,9 @@ int ubiblock_create(struct ubi_volume_info *vi)
 	}
 
 	dev->rq = blk_mq_init_queue(&dev->tag_set);
-	if (!dev->rq) {
+	if (IS_ERR(dev->rq)) {
 		dev_err(disk_to_dev(gd), "blk_mq_init_queue failed");
-		ret = -ENODEV;
+		ret = PTR_ERR(dev->rq);
 		goto out_free_tags;
 	}
 	blk_queue_max_segments(dev->rq, UBI_MAX_SG_COUNT);