ekf2_main: reduce update mag bias minimum learning time to 30s

120 seconds of learning time in 3D fusion mode was too long for most normal flights. The learned bias is usually really good after a shorter period and was not used to update the parameters. 30s seems to be a good compromise.

src/modules/ekf2/EKF2.cpp

@@ -1131,7 +1131,7 @@ void EKF2::Run()
 				}
 
 				// Start checking mag bias estimates when we have accumulated sufficient calibration time
-				if (_total_cal_time_us > 120_s) {
+				if (_total_cal_time_us > 30_s) {
 					// we have sufficient accumulated valid flight time to form a reliable bias estimate
 					// check that the state variance for each axis is within a range indicating filter convergence
 					const float max_var_allowed = 100.0f * _param_ekf2_magb_vref.get();