mac80211_hwsim: suggest nl80211 instead of wext driver in documentation

For mac80211_hwsim interfaces, suggest to use wpa_supplicant with the more
modern, netlink based driver instead of wext.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>