net: thunderx: mark expected switch fall-throughs in nicvf_main()

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

Cc: Sunil Goutham <sgoutham@cavium.com>
Cc: Robert Richter <rric@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org
Cc: netdev@vger.kernel.org
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>