net80211: refactor the if_input call into ieee80211_vap_deliver_data()

Refactor the two places where NET_EPOCH_ENTER; if_input; NET_EPOCH_EXIT
are called into a single spot in ieee80211_freebsd.c.

This removes both if_input references and puts all the NET_EPOCH stuff
into ieee80211_freebsd.c.

Differential Revision: https://reviews.freebsd.org/D50404
Reviewed by: bz