isdn/gigaset: unify function return values

Various functions in the Gigaset driver were using different
conventions for the meaning of their int return values.
Align them to the usual negative error numbers convention.

Inspired-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>