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

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

The "brcm" and "brcm-prepend" 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. The port mask is written u8 by u8,
first the high octet and then the low octet.

Cc: Florian Fainelli <florian.fainelli@broadcom.com>
Cc: Jonas Gorski <jonas.gorski@gmail.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://patch.msgid.link/20251127120902.292555-3-vladimir.oltean@nxp.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>