AP_HAL_Linux: RCInput: replace volatile with atomic

libraries/AP_HAL_Linux/RCInput.cpp

@@ -38,7 +38,7 @@ bool RCInput::new_input()
 {
     bool ret = rc_input_count != last_rc_input_count;
     if (ret) {
-        last_rc_input_count = rc_input_count;
+        last_rc_input_count.store(rc_input_count);
     }
     return ret;
 }

libraries/AP_HAL_Linux/RCInput.h

@@ -1,5 +1,7 @@
 #pragma once
 
+#include <atomic>
+
 #include "AP_HAL_Linux.h"
 
 #define LINUX_RC_INPUT_NUM_CHANNELS 16
@@ -42,13 +44,13 @@ public:
 
     // add some srxl input bytes, for RCInput over a serial port
     bool add_srxl_input(const uint8_t *bytes, size_t nbytes);
-    
+
 protected:
     void _process_rc_pulse(uint16_t width_s0, uint16_t width_s1);
     void _update_periods(uint16_t *periods, uint8_t len);
 
-    volatile uint32_t rc_input_count;
-    uint32_t last_rc_input_count;
+    std::atomic<unsigned int> rc_input_count;
+    std::atomic<unsigned int> last_rc_input_count;
 
     uint16_t _pwm_values[LINUX_RC_INPUT_NUM_CHANNELS];
     uint8_t  _num_channels;