net: mscc: ocelot: use helpers for port VLAN membership

This is a mostly cosmetic patch that creates some helpers for accessing
the VLAN table. These helpers are also a bit more careful in that they
do not modify the ocelot->vlan_mask unless the hardware operation
succeeded.

Not all callers check the return value (the init code doesn't), but anyway.

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