sensors/vehicle_gps_position: Fix raw_dt and present_dt calculation in gps_blending

If the GPS data is passed from companion computer, there may be inaccuracies
with timesync. This may cause time deltas to overflow.

Make the time delta calculation using floats directly, wich results in negative
numbers in case there are such inaccuracies.

Signed-off-by: Jukka Laitinen <jukkax@ssrc.tii.ae>

src/modules/sensors/vehicle_gps_position/gps_blending.cpp

@@ -105,8 +105,8 @@ bool GpsBlending::blend_gps_data(uint64_t hrt_now_us)
 	_np_gps_suitable_for_blending = 0;
 
 	for (uint8_t i = 0; i < GPS_MAX_RECEIVERS_BLEND; i++) {
-		const float raw_dt = 1e-6f * (float)(_gps_state[i].timestamp - _time_prev_us[i]);
-		const float present_dt = 1e-6f * (float)(hrt_now_us - _gps_state[i].timestamp);
+		const float raw_dt = 1e-6f * ((float)_gps_state[i].timestamp - (float)_time_prev_us[i]);
+		const float present_dt = 1e-6f * ((float)hrt_now_us - (float)_gps_state[i].timestamp);
 
 		if (raw_dt > 0.0f && raw_dt < GPS_TIMEOUT_S) {
 			_gps_dt[i] = 0.1f * raw_dt + 0.9f * _gps_dt[i];