Move anti-windup iterm reset from rx loop to pid loop

Having the iterm resetting happening in the rx loop causes a sawtooth PID/motor effect while idling since the PID loop is running at a much higher rate and iterm is allowed to grow during this, and then only reset at a much lower rate in the rx loop. This can potentially lead to some oscillation and/or resonance while idling before takeoff as the sawtooth signal can make it through to the motor outputs.

src/test/unit/vtx_unittest.cc

@@ -174,4 +174,5 @@ extern "C" {
     bool usbVcpIsConnected(void) { return false; }
     void pidSetAntiGravityState(bool newState) { UNUSED(newState); }
     void osdSuppressStats(bool) {}
+    void pidSetItermReset(bool) {}
 }