net80211: make ieee80211_alloc_node() private

Looking through the allocation/free and reference cycle of nodes we have
a few (publicly) (almost) unused macros and functions.
Start making them private and reducing the amount of entry paths to the
same/similar functionality.
First is to make ieee80211_alloc_node() static as it is nowhere used
in the tree outside this file.

Sponsored by: The FreeBSD Foundation
X-MFC: no