gigaset: allow building without I4L

Add a dummy LL interface to the Gigaset driver so that it can be
built and, in a limited way, used without the ISDN4Linux subsystem.

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