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" <[e-mail]>
Cc: Daniel Golle <[e-mail]>
Cc: DENG Qingfang <[e-mail]>
Cc: Sean Wang <[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_mtk.c

···
 	 * 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) {