EKF: Fix use of incorrect timestamp

This was incorrectly using the IMU (1/250 sec) timestamp instead of the EKF (1/100 sec) value.
The corresponding accelerometer limit has been made a parameter and adjusted to match previous behaviour.

EKF/common.h

@@ -269,6 +269,9 @@ struct parameters {
 	float vel_Tau;	// velocity state correction time constant (1/sec)
 	float pos_Tau;	// postion state correction time constant (1/sec)
 
+	// state limits
+	float acc_bias_lim;	// maximum accel bias magnitude (m/s/s)
+
 	unsigned no_gps_timeout_max;	// maximum time we allow dead reckoning while both gps position and velocity measurements are being
 									// rejected
 
@@ -373,6 +376,9 @@ struct parameters {
 		vel_Tau = 0.25f;
 		pos_Tau = 0.25f;
 
+		// state limiting
+		acc_bias_lim = 0.4f;
+
 		no_gps_timeout_max = 7e6;	// maximum seven seconds of dead reckoning time for gps
 
 	}