net: dsa: tag_rtl8_4: use the dsa_xmit_port_mask() helper

net: dsa: tag_rtl8_4: use the dsa_xmit_port_mask() helper

The "rtl8_4" and "rtl8_4t" tagging protocols populate 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/20251127120902.292555-12-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>