ACM: make it possible to set AHRS_GPS_GAIN on ArduCopter

this was affected by the AP_Param change, in particular the
constructor ordering. To ensure a user can set AHRS_GPS_GAIN to 1.0 if
they want to, we need to do a set_and_save() if the value isn't in
EEPROM

ArduCopter/Parameters.pde

@@ -358,7 +358,10 @@ const AP_Param::Info var_info[] PROGMEM = {
 static void load_parameters(void)
 {
 	// change the default for the AHRS_GPS_GAIN for ArduCopter
-	ahrs.gps_gain.set(0.0);
+	// if it hasn't been set by the user
+	if (!ahrs.gps_gain.load()) {
+		ahrs.gps_gain.set_and_save(0.0);
+	}
 
 	if (!g.format_version.load() ||
 	    g.format_version != Parameters::k_format_version) {