UBIFS: return proper error code if the compr is not present

If the compressor is not present, mount_ubifs need
to return an error code. This way ubifs_fill_super
will stop and handle the error.

Signed-off-by: Corentin Chary <corentincj@iksaif.net>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>