gigaset: fix build failure

Update the dummy LL interface to the LL interface change
introduced by commit daab433c03c15fd642c71c94eb51bdd3f32602c8.
This fixes the build failure occurring after that commit when
enabling ISDN_DRV_GIGASET but neither ISDN_I4L nor ISDN_CAPI.

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