net: plip: mark expected switch fall-throughs

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

Addresses-Coverity-ID: 114893
Addresses-Coverity-ID: 114894
Addresses-Coverity-ID: 114895
Addresses-Coverity-ID: 114896
Addresses-Coverity-ID: 114897
Addresses-Coverity-ID: 114898
Addresses-Coverity-ID: 114899
Addresses-Coverity-ID: 114900
Addresses-Coverity-ID: 114901
Addresses-Coverity-ID: 114902
Addresses-Coverity-ID: 114903
Addresses-Coverity-ID: 114904
Addresses-Coverity-ID: 114905
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>