selftests: net: mscc: ocelot: rename the VLAN modification test to ingress

There will be one more VLAN modification selftest added, this time for
egress. Rename the one that exists right now to be more specific.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>