net: dsa: sja1105: keep the VLAN awareness state in a driver variable

Soon we'll add a third operating mode to the driver. Introduce a
vlan_state to make things more easy to manage, and use it where
applicable.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>