HAL_Linux: UARTDriver, const device_path

Class member set to const and set_device_path
accepts a const char* now.

libraries/AP_HAL_Linux/UARTDriver.cpp

@@ -45,7 +45,7 @@ LinuxUARTDriver::LinuxUARTDriver(bool default_console) :
 /*
   set the tty device to use for this UART
  */
-void LinuxUARTDriver::set_device_path(char *path)
+void LinuxUARTDriver::set_device_path(const char *path)
 {
     device_path = path;
 }

libraries/AP_HAL_Linux/UARTDriver.h

@@ -25,7 +25,7 @@ public:
     size_t write(uint8_t c);
     size_t write(const uint8_t *buffer, size_t size);
 
-    void set_device_path(char *path);
+    void set_device_path(const char *path);
 
     virtual void _timer_tick(void);
 
@@ -58,7 +58,7 @@ private:
     uint64_t _last_write_time;    
 
 protected:
-    char *device_path;
+    const char *device_path;
     volatile bool _initialised;
     // we use in-task ring buffers to reduce the system call cost
     // of ::read() and ::write() in the main loop