WPS: Set currently used RF band in RF Bands attribute

WPS: Set currently used RF band in RF Bands attribute

According to WSC specification (Ver 2.0.2, section 8.3), RF Bands
attribute should be set to the specific RF band used for the current
message. Add an option to set wanted band in wps_build_rf_bands() and
add a callback to get the current band from wpa_supplicant and hostapd.

Signed-hostap: David Spinadel <david.spinadel@intel.com>