mac80211: always use wpa_supplicant for client connections (patch by Stijn Tintel)

mac80211: always use wpa_supplicant for client connections (patch by Stijn Tintel)

Since client mode in mac80211is unreliable without using wpa_supplicant,
it would be better to enable wpa_supplicant by default.

With this patch, wpa_supplicant will always be used for mac80211-based
drivers in client mode. This will break client mode for images that
don't include wpa_supplicant or wpad, so maybe I need to add a check
somewhere to see if wpa_supplicant is available, and show the user a
warning if he tries client mode without wpa_supplicant. Suggestions on
where to add this check are welcome.

Tested with ath5k in client mode on open and wpa2-psk network, and
tested on my ath9k based AP as well to make sure it doesn't break AP mode.

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@19718 3c298f89-4303-0410-b956-a3cf2f4a3e73