net: dsa: tag_rtl4_a: use the dsa_xmit_port_mask() helper

The "rtl4a" 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: Linus Walleij <linus.walleij@linaro.org>
Cc: "Alvin Šipraga" <alsi@bang-olufsen.dk>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251127120902.292555-11-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: vladimiroltean

net/dsa/tag_rtl4_a.c

@@ -57,7 +57,7 @@ static struct sk_buff *rtl4a_tag_xmit(struct sk_buff *skb,
 
 	out = (RTL4_A_PROTOCOL_RTL8366RB << RTL4_A_PROTOCOL_SHIFT);
 	/* The lower bits indicate the port number */
-	out |= BIT(dp->index);
+	out |= dsa_xmit_port_mask(skb, dev);
 
 	p = (__be16 *)(tag + 2);
 	*p = htons(out);