LinuxKPI: 802.11: avoid symbol clash on UP to AC mapping

tid_to_mac80211_ac is an exported symbol in and likely based on iwlwifi,
which leads to a symbol clash in NetBSD. Rename our local LinuxKPI copy
to a better name and add a comment where to find a copy of the mapping
table.

Sponsored by: The FreeBSD Foundation
MFC after: 3 days
Reported by: Phil Nelson (phil netbsd org)