net: dsa: tag_sja1105: refactor skb->dev assignment to dsa_tag_8021q_find_user()

net: dsa: tag_sja1105: refactor skb->dev assignment to dsa_tag_8021q_find_user()

A new tagging protocol implementation based on tag_8021q is on the
horizon, and it appears that it also has to open-code the complicated
logic of finding a source port based on a VLAN header.

Create a single dsa_tag_8021q_find_user() and make sja1105 call it.

Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Link: https://patch.msgid.link/20240713211620.1125910-7-paweldembicki@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>