wpa_supplicant: Default to nl80211 instead of wext

wpa_supplicant: Default to nl80211 instead of wext

nl80211 has obsoleted WEXT as the preferred kernel interface for
controlling wireless drivers. Update wpa_supplicant driver interface
list order so that nl80211 gets used first if both nl80211 and wext
interfaces are included in the build. In addition, update README to
reflect the fact that WEXT is obsolete.

Signed-hostap: Jouni Malinen <jouni@qca.qualcomm.com>