NavEKF: IMU ratio set to primary accel when neither IMU is used

This forces the EKF IMU ratio to indicate which IMU is used except that it will be "0" in the unlikely case that the third IMU is used

libraries/AP_NavEKF/AP_NavEKF.cpp

@@ -4187,7 +4187,11 @@ void NavEKF::readIMUData()
             lastImuSwitchState = 2;
         } else {
             readDeltaVelocity(ins.get_primary_accel(), dVelIMU1, dtDelVel1);
-            lastImuSwitchState = 0;
+            if (ins.get_primary_accel() < 2) {
+                lastImuSwitchState = ins.get_primary_accel() + 1;
+            } else {
+                lastImuSwitchState = 0;
+            }
         }
         dtDelVel2 = dtDelVel1;
         dVelIMU2 = dVelIMU1;