net80211 / drivers: remove public use of ieee80211_node_incref()

ieee80211_node_incref() is the FreeBSD implementation of
ieee80211_ref_node(). Not being interested in the node returned
it was used as a shortcut in 3 drivers (ath, uath, wpi).
Replace the call with the public KPI of ieee80211_ref_node() and
ignore the result.
This leaves us with the single internal call going
ieee80211_ref_node() -> ieee80211_node_incref() and that should
help increasing portability but also limiting the places to trace
for node reference operations.

Sponsored by: The FreeBSD Foundation
MFC after: 4 weeks