net: dsa: tag_gswip: use the dsa_xmit_port_mask() helper

The "gswip" tagging protocol populates a bit mask for the TX ports, so
we can use dsa_xmit_port_mask() to centralize the decision of how to set
that field.

Cc: Hauke Mehrtens <[e-mail]>
Cc: Daniel Golle <[e-mail]>
Signed-off-by: Vladimir Oltean <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

net/dsa/tag_gswip.c

···
 
 /* Byte 3 */
 #define GSWIP_TX_DPID_EN		BIT(0)
-#define GSWIP_TX_PORT_MAP_SHIFT		1
-#define GSWIP_TX_PORT_MAP_MASK		GENMASK(6, 1)
+#define GSWIP_TX_PORT_MAP		GENMASK(6, 1)
 
 #define GSWIP_RX_HEADER_LEN	8
 
···
 static struct sk_buff *gswip_tag_xmit(struct sk_buff *skb,
 				      struct net_device *dev)
 {
-	struct dsa_port *dp = dsa_user_to_port(dev);
 	u8 *gswip_tag;
 
 	skb_push(skb, GSWIP_TX_HEADER_LEN);
···
 	gswip_tag[0] = GSWIP_TX_SLPID_CPU;
 	gswip_tag[1] = GSWIP_TX_DPID_ELAN;
 	gswip_tag[2] = GSWIP_TX_PORT_MAP_EN | GSWIP_TX_PORT_MAP_SEL;
-	gswip_tag[3] = BIT(dp->index + GSWIP_TX_PORT_MAP_SHIFT) & GSWIP_TX_PORT_MAP_MASK;
+	gswip_tag[3] = FIELD_PREP(GSWIP_TX_PORT_MAP, dsa_xmit_port_mask(skb, dev));
 	gswip_tag[3] |= GSWIP_TX_DPID_EN;
 
 	return skb;