ISDN: eicon: message: mark expected switch fall-throughs

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

Addresses-Coverity-ID: 114780
Addresses-Coverity-ID: 114781
Addresses-Coverity-ID: 114782
Addresses-Coverity-ID: 114783
Addresses-Coverity-ID: 114784
Addresses-Coverity-ID: 114785
Addresses-Coverity-ID: 114786
Addresses-Coverity-ID: 114787
Addresses-Coverity-ID: 114788
Addresses-Coverity-ID: 114789
Addresses-Coverity-ID: 114790
Addresses-Coverity-ID: 114791
Addresses-Coverity-ID: 114792
Addresses-Coverity-ID: 114793
Addresses-Coverity-ID: 114794
Addresses-Coverity-ID: 114795
Addresses-Coverity-ID: 200521
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>