net: dp83640: 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 code comment at the
top of the switch statement with a proper "fall through" annotation for
each case, which is what GCC is expecting to find.

Addresses-Coverity-ID: 1056542 ("Missing break in switch")
Addresses-Coverity-ID: 1339579 ("Missing break in switch")
Addresses-Coverity-ID: 1369526 ("Missing break in switch")
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>