isdn/gigaset: missing break in do_facility_req

If we take the unsupported supplementary service notification mask
path, we end up falling through and overwriting the error code.
Insert a break statement to skip the remainder of the switch case
and proceed to sending the reply message.

Spotted with Coverity.

Reported-by: Dave Jones <davej@redhat.com>
Signed-off-by: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: David S. Miller <davem@davemloft.net>