hostapd: Allow enabling background radar

This feature does not work on all radios that advertise this feature
with the current driver implementation, and possibly some users don't
want to use it even if it works fine, so disable it by default for now,
but let users enable it as desired with enable_background_radar=1.

Signed-off-by: Ben Greear <greearb@candelatech.com>

src/ap/ap_config.h

@@ -969,6 +969,7 @@ struct hostapd_config {
 	u8 min_tx_power;
 	enum hostapd_hw_mode hw_mode; /* HOSTAPD_MODE_IEEE80211A, .. */
 	int acs_exclude_6ghz_non_psc;
+	int enable_background_radar;
 	enum {
 		LONG_PREAMBLE = 0,
 		SHORT_PREAMBLE = 1