isdn/gigaset: mark expected switch fall-throughs

Replace "--v-- fall through --v--" with a proper "fall through"
annotation. Also, change "bad cid: fall through" to
"fall through - bad cid".

This fix is part of the ongoing efforts to enabling -Wimplicit-fallthrough

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>