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>