net80211: enable promiscuous mode state change for non-monitor/ahdemo modes

• Allow to enable/disable promiscuous mode when:
  • interface is not a member of bridge, or;
  • request was issued by user (ifconfig wlan0 promisc), or;
  • interface is in MONITOR or AHDEMO mode.
• Drop local workarounds in mwl(4) and malo(4).

Tested with:

• Intel 3945BG, STA mode;
• RTL8188CUS, MONITOR mode;

Reviewed by: adrian
Differential Revision: https://reviews.freebsd.org/D5472