AC_WPNav: bug fix for loiter init in Hybrid

AC_PosControl::init_xy_controller() has been added to PosControl and is
called by init_loiter_target.
Hybrid is currently using set_loiter_target function to init the loiter
controller. So we have to call init_xy_controller() by set_loiter_target
function.
What happens otherwise?
In AC_PosControl::update_xy_controller, we update "now" with
now = hal.scheduler->millis();
and, as _last_update_xy_ms has not been updated previously by
init_xy_controller(), we just call init_xy_controller().
So, _dt_xy  will be negative and used anyways in all the functions and
PID called by update_xy_controller.
That will avoid at least _accel_target.x/y to be set to 0 but I'm not
sure for the high values, probably an I_term that is not reset and
reached very high value.
Or maybe a cast error somewhere... no clue at all

libraries/AC_WPNav/AC_WPNav.cpp

@@ -121,6 +121,9 @@ void AC_WPNav::set_loiter_target(const Vector3f& position, bool reset_I)
     if (!reset_I) {
         _pos_control.keep_xy_I_terms();
     }
+    
+    // initialise position controller
+    _pos_control.init_xy_controller();
 
     // set target position
     _pos_control.set_pos_target(position);
@@ -135,6 +138,10 @@ void AC_WPNav::set_loiter_target(const Vector3f& position, bool reset_I)
     _loiter_accel_cms = _loiter_speed_cms/2.0f;
     _pos_control.set_accel_xy(_loiter_accel_cms);
 
+    // initialise desired accel and add fake wind
+    _loiter_desired_accel.x = 0;
+    _loiter_desired_accel.y = 0;
+
     // initialise pilot input
     _pilot_accel_fwd_cms = 0;
     _pilot_accel_rgt_cms = 0;