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

The "hellcreek" 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: Kurt Kanzenbach <[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_hellcreek.c

···
 static struct sk_buff *hellcreek_xmit(struct sk_buff *skb,
 				      struct net_device *dev)
 {
-	struct dsa_port *dp = dsa_user_to_port(dev);
 	u8 *tag;
 
 	/* Calculate checksums (if required) before adding the trailer tag to
···
 
 	/* Tag encoding */
 	tag  = skb_put(skb, HELLCREEK_TAG_LEN);
-	*tag = BIT(dp->index);
+	*tag = dsa_xmit_port_mask(skb, dev);
 
 	return skb;
 }