isdn/gigaset/isocdata: mark expected switch fall-through

Notice that in this particular case, I replaced the
"--v-- fall through --v--" comment with a proper
"fall through", which is what GCC is expecting to
find.

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>