net80211: migrate the group/unicast key check into inline functions

The way that net80211 and drivers are checking for the /type/ of key
is to check if it's in the vap WEP key array and if so, it's a group
key. If not, it's a unicast key.

That's not only kind of terrible, but it's also going to be
problematic with future 802.11 support (for multiple unicast keys
and IGTK keys for management frame protection.)

So as part of this, remove the places where this is done and
instead use a pair inline functions - ieee80211_is_key_global() and
ieee80211_is_key_unicast(). They currenly still use the same logic
but the drivers and net80211 stack isn't doing it itself.

There are still open questions about why keys are not being
correctly tagged as GROUP, GTK, PTK, etc. That will be investigated
and addressed in follow-up work as a pre-cursor to MFP, IGTK, etc.
as mentioned above.

Testing:

• iwn, rtwn - STA mode

Differential Revision: https://reviews.freebsd.org/D45516