When using a cli-based RATP that opens and closes the RATP channel for
each command sent/received, it gets annoying to get the "Closed"
message printed all the time in console after each RATP channel close
operation. Just demote it to debug level.

E.g.:
    barebox@barebox sandbox:/ ratp: Closed
    ratp: Closed
    ratp: Closed
    ratp: Closed
    ...

Signed-off-by: Aleksander Morgado <aleksander@aleksander.es>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>