isdn/gigaset: cede maintainership

As German phone operators are discontinuing ISDN service, neither
Hansjörg nor I will be able to maintain the Gigaset ISDN drivers
any longer. Paul Bolle offered to step into the breach for odd
fixes.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Acked-by: Paul Bolle <pebolle@tiscali.nl>
Signed-off-by: David S. Miller <davem@davemloft.net>