isdn/gigaset: ignore irrelevant device responses

Downgrade the Gigaset driver's reaction to unknown AT responses from
the device from warning to debug level, and remove the handling of
some device responses which aren't relevant for the driver's
operation.

Impact: cleanup
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>