isdn/gigaset: report ISDN4Linux interface only once

Move the "ISDN4Linux interface" message from device registration,
where it is emitted for each device, to driver registration, where
it is emitted only once, for consistency with the CAPI variant.

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