WPS: Make it possible to use PSKs loaded from the PSK file

WPS: Make it possible to use PSKs loaded from the PSK file

By default, when configuration file set wpa_psk_file, hostapd generated
a random PSK for each Enrollee provisioned using WPS and appended that
PSK to wpa_psk_file.

Changes that behavior by adding a new step. WPS will first try to use a
PSK from wpa_psk_file. It will only try PSKs with wps=1 tag.
Additionally it'll try to match enrollee's MAC address (if provided). If
it fails to find an appropriate PSK, it falls back to generating a new
PSK.

Signed-off-by: Tomasz Jankowski <tomasz.jankowski@plume.com>