AP_HAL_SITL: correct disable channel maths

unilaterally write rcoutput to appease the sitl gods

libraries/AP_HAL_SITL/RCOutput.cpp

@@ -35,7 +35,7 @@ void RCOutput::enable_ch(uint8_t ch)
     if (!(_enable_mask & (1U << ch))) {
         Debug("enable_ch(%u)\n", ch);
     }
-    _enable_mask |= 1U << ch;
+    _enable_mask |= (1U << ch);
 }
 
 void RCOutput::disable_ch(uint8_t ch)
@@ -43,13 +43,14 @@ void RCOutput::disable_ch(uint8_t ch)
     if (_enable_mask & (1U << ch)) {
         Debug("disable_ch(%u)\n", ch);
     }
-    _enable_mask &= ~1U << ch;
+    _enable_mask &= ~(1U << ch);
 }
 
 void RCOutput::write(uint8_t ch, uint16_t period_us)
 {
     _sitlState->output_ready = true;
-    if (ch < SITL_NUM_CHANNELS && (_enable_mask & (1U<<ch))) {
+    // FIXME: something in sitl is expecting to be able to read and write disabled channels
+    if (ch < SITL_NUM_CHANNELS /*&& (_enable_mask & (1U<<ch))*/) {
         if (_corked) {
             _pending[ch] = period_us;
         } else {
@@ -60,7 +61,8 @@ void RCOutput::write(uint8_t ch, uint16_t period_us)
 
 uint16_t RCOutput::read(uint8_t ch)
 {
-    if (ch < SITL_NUM_CHANNELS) {
+    // FIXME: something in sitl is expecting to be able to read and write disabled channels
+    if (ch < SITL_NUM_CHANNELS /*&& (_enable_mask & (1U<<ch))*/) {
         return _sitlState->pwm_output[ch];
     }
     return 0;