fs/erofs: silence erofs_probe()

fs_set_blk_dev() probes all file-systems until it finds one that matches
the volume. We do not expect any console output for non-matching
file-systems.

Convert error messages in erofs_read_superblock() to debug output.

Fixes: 830613f8 ("fs/erofs: add erofs filesystem support")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Huang Jianan <jnhuang95@gmail.com>