net: dsa: tag_sja1105: replace skb_mac_header() with vlan_eth_hdr()

This is a cosmetic patch which consolidates the code to use the helper
function offered by if_vlan.h.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/dsa/tag_sja1105.c

@@ -516,7 +516,7 @@ static bool sja1110_skb_has_inband_control_extension(const struct sk_buff *skb)
 static void sja1105_vlan_rcv(struct sk_buff *skb, int *source_port,
 			     int *switch_id, int *vbid, u16 *vid)
 {
-	struct vlan_ethhdr *hdr = (struct vlan_ethhdr *)skb_mac_header(skb);
+	struct vlan_ethhdr *hdr = vlan_eth_hdr(skb);
 	u16 vlan_tci;
 
 	if (skb_vlan_tag_present(skb))