wext: use combo scan to avoid SIOCSIWSCAN error

wpa_supplicant can't start scan, logcat info:
W/wpa_supplicant( 1936): eth0: Failed to initiate AP scan
I/wpa_supplicant( 1936): ioctl[SIOCSIWSCAN]: Invalid argument

Change-Id: If81c48b782ba0e7d42bc2ed7a95166b29688354a