net: 3c509: mark expected switch fall-throughs

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

This patch fixes the following warnings:

drivers/net/ethernet/3com/3c509.c:1265:8: warning: this statement may fall through [-Wimplicit-fallthrough=]
drivers/net/ethernet/3com/3c509.c:1271:8: warning: this statement may fall through [-Wimplicit-fallthrough=]

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch 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>