gigaset: asyncdata: mark expected switch fall-throughs

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to 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.

Addresses-Coverity-ID: 1364476 ("Missing break in switch")
Addresses-Coverity-ID: 1364477 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>