isdn/gigaset: enable Kernel CAPI support by default

Kernel CAPI has been the recommended ISDN subsystem for the Gigaset
driver since kernel release 2.6.34.2. It provides full backwards
compatibility to the old I4L subsystem thanks to the capidrv module.
I4L has been marked as deprecated for more than seven years.

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