gigaset: move up Kconfig inclusion point

In preparation for porting to kernel CAPI subsystem, include the
Gigaset driver's Kconfig directly from ISDN's instead of I4L's.

Impact: Kconfig reorganisation, no functional change
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>