HAL_Linux: change the return type of AP_HAL::Util::tonealarm_init() to signed int8_t.

This is a part of a set of fixes for the bugs and typos tridge discovered and shared inside earlier commits for setting up tonealarm.

libraries/AP_HAL/Util.h

@@ -62,7 +62,7 @@ public:
     /*
         ToneAlarm Driver
     */
-    virtual uint8_t toneAlarm_init() { return -1;}
+    virtual int8_t toneAlarm_init() { return -1;}
     virtual void toneAlarm_set_tune(uint8_t tune) {}
     virtual void _toneAlarm_timer_tick() {}
     

libraries/AP_HAL_Linux/ToneAlarmDriver.cpp

@@ -57,7 +57,7 @@ void LinuxUtil::toneAlarm()
     
     tune_num = -1;                    //initialy no tune to play
 }
-uint8_t LinuxUtil::toneAlarm_init()
+int8_t LinuxUtil::toneAlarm_init()
 {
     tune_num = 0;                    //play startup tune
     if((period_fd == -1) || (duty_fd == -1) || (run_fd == -1)){

libraries/AP_HAL_Linux/Util.h

@@ -127,7 +127,7 @@ public:
      */
     void commandline_arguments(uint8_t &argc, char * const *&argv);
 
-    uint8_t toneAlarm_init();
+    int8_t toneAlarm_init();
     void toneAlarm_set_tune(uint8_t tune);
     
     void _toneAlarm_timer_tick();