net80211: fix possible panic when wlan(4) interface is destroyed.

If this is the last running vap wait until device will be powered off
(fixes panic when 'ifconfig wlan0 destroy' is executed for running iwn(4)
interface).

Tested with:

• Intel 6205, STA mode.
• RTL8188EU, STA / IBSS modes.
• RTL8821AU, STA / HOSTAP modes.