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

The "mtk" 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: Chester A. Unal" <chester.a.unal@arinc9.com>
Cc: Daniel Golle <daniel@makrotopia.org>
Cc: DENG Qingfang <dqfext@gmail.com>
Cc: Sean Wang <sean.wang@mediatek.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251127120902.292555-7-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

net/dsa/tag_mtk.c

@@ -54,7 +54,8 @@ static struct sk_buff *mtk_tag_xmit(struct sk_buff *skb,
 	 * whether that's a combined special tag with 802.1Q header.
 	 */
 	mtk_tag[0] = xmit_tpid;
-	mtk_tag[1] = (1 << dp->index) & MTK_HDR_XMIT_DP_BIT_MASK;
+	mtk_tag[1] = FIELD_PREP(MTK_HDR_XMIT_DP_BIT_MASK,
+				dsa_xmit_port_mask(skb, dev));
 
 	/* Tag control information is kept for 802.1Q */
 	if (xmit_tpid == MTK_HDR_XMIT_UNTAGGED) {