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

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>