ubi: Lower 'already attached' message to debug level
The caller will show a message, no need to do this in UBI. This
is for the case when mtd_detect calls ubi_attach_mtd_dev. mtd_detect
does not know whether this is already attached and it's not an error.
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>