isdn/gigaset: fix CAPI disconnect B3 handling

If DISCONNECT_B3_IND was synthesized because of a DISCONNECT_REQ
with existing logical connections, the connection state wasn't
updated accordingly. Also the emitted DISCONNECT_B3_IND message
wasn't included in the debug log as requested.
This patch fixes both of these issues.

Signed-off-by: Tilman Schmidt <tilman@imap.cc>
CC: stable <stable@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>