net: dsa: Obtain VLAN protocol from skb->protocol

Now that dsa_untag_bridge_pvid() is called after eth_type_trans() we are
guaranteed that skb->protocol will be set to a correct value, thus
allowing us to avoid calling vlan_eth_hdr().

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