fs/erofs: Quieten test for filesystem presence

At present listing a partition produces lots of errors about this
filesystem:

   => part list mmc 4
   cannot find valid erofs superblock
   cannot find valid erofs superblock
   cannot read erofs superblock: -5
   [9 more similar lines]

Use debugging rather than errors when unable to find a signature, as is
done with other filesystems.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/fs/erofs/super.c b/fs/erofs/super.c
index d339262..d405d48 100644
--- a/fs/erofs/super.c
+++ b/fs/erofs/super.c
@@ -68,14 +68,14 @@
 
 	ret = erofs_blk_read(data, 0, erofs_blknr(sizeof(data)));
 	if (ret < 0) {
-		erofs_err("cannot read erofs superblock: %d", ret);
+		erofs_dbg("cannot read erofs superblock: %d", ret);
 		return -EIO;
 	}
 	dsb = (struct erofs_super_block *)(data + EROFS_SUPER_OFFSET);
 
 	ret = -EINVAL;
 	if (le32_to_cpu(dsb->magic) != EROFS_SUPER_MAGIC_V1) {
-		erofs_err("cannot find valid erofs superblock");
+		erofs_dbg("cannot find valid erofs superblock");
 		return ret;
 	}