net80211: remove redundant locking.

All callers of ieee80211_promisc()/ieee80211_allmulti()
(ieee80211_vap_detach(), ieee80211_ioctl(), ap_start() and ap_end())
already hold the com_lock while calling them.

Tested with RTL8188EU, STA mode.

Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D5475