WPS: Fix return value when context is not valid

WPS: Fix return value when context is not valid

If WPS isn't enabled, hostapd_cli returns 'OK' even though WPS doesn't
get activated because WPS context is not valid:

$ hostapd_cli wps_pbc
Selected interface 'wlan0'
OK

$ hostapd_cli wps_cancel
Selected interface 'wlan0'
OK

Fix this by returning appropriate error when WPS fails to activate:

$ hostapd_cli wps_pbc
Selected interface 'wlan0'
FAIL

$ hostapd_cli wps_cancel
Selected interface 'wlan0'
FAIL

Signed-off-by: Petar Koretic <petar.koretic@sartura.hr>
CC: Luka Perkov <luka.perkov@sartura.hr>