ekf: clean uncorrelateQuatStates function

As the name can be ambiguous, it gets renamed
"uncorrelateQuatFromOtherStates".
Also replace the loops storing the values and reapplying them by simply
zeroing two slices (this also saves 130B of flash).

EKF/control.cpp

@@ -227,7 +227,7 @@ void Ekf::controlExternalVisionFusion()
 
 				// calculate initial quaternion states for the ekf
 				_state.quat_nominal = Quatf(euler_init);
-				uncorrelateQuatStates();
+				uncorrelateQuatFromOtherStates();
 
 				// adjust the quaternion covariances estimated yaw error
 				increaseQuatYawErrVariance(fmaxf(_ev_sample_delayed.angVar, sq(1.0e-2f)));