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 <[e-mail]>
Cc: "Alvin Šipraga" <[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_rtl4_a.c

···
 
 	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);