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

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