net: mscc: ocelot: Fix fall-through warnings for Clang

In preparation to enable -Wimplicit-fallthrough for Clang, fix a warning
by explicitly adding a break statement instead of just letting the code
fall through to the next case.

Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://github.com/KSPP/linux/issues/115
Signed-off-by: David S. Miller <davem@davemloft.net>