gigaset: prepare for CAPI implementation

Reorganize the code of the Gigaset driver, moving all isdn4linux
dependencies to the source file i4l.c so that it can be replaced
by a file capi.c interfacing to Kernel CAPI instead.

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