ekf2-bias_cal: reduce required stability period

For the mag, 30s of bias learning is a lot, given that it is only active
during turns and that it usually converges in < 5 seconds

src/modules/ekf2/EKF2.cpp

@@ -2461,7 +2461,7 @@ void EKF2::UpdateCalibration(const hrt_abstime &timestamp, InFlightCalibration &
 				cal.total_time_us += timestamp - cal.last_us;
 			}
 
-			if (cal.total_time_us > 30_s) {
+			if (cal.total_time_us > 10_s) {
 				cal.cal_available = true;
 			}