ArduCopter: Do not use is_zero() for non-float types

This function makes only sense for floating point types. However this
function was also used for ints.

ArduCopter/control_autotune.cpp

@@ -307,7 +307,7 @@ void Copter::autotune_run()
         pos_control.relax_alt_hold_controllers(get_throttle_pre_takeoff(channel_throttle->get_control_in())-throttle_average);
     }else{
         // check if pilot is overriding the controls
-        if (!is_zero(target_roll) || !is_zero(target_pitch) || !is_zero(target_yaw_rate) || !is_zero(target_climb_rate)) {
+        if (!is_zero(target_roll) || !is_zero(target_pitch) || !is_zero(target_yaw_rate) || target_climb_rate != 0) {
             if (!autotune_state.pilot_override) {
                 autotune_state.pilot_override = true;
                 // set gains to their original values

ArduCopter/control_rtl.cpp

@@ -97,7 +97,7 @@ void Copter::rtl_climb_start()
     wp_nav.wp_and_spline_init();
 
     // RTL_SPEED == 0 means use WPNAV_SPEED
-    if (!is_zero(g.rtl_speed_cms)) {
+    if (g.rtl_speed_cms != 0) {
         wp_nav.set_speed_xy(g.rtl_speed_cms);
     }