AP_Notify: cleanup use of bool from toneAlarm_init()

libraries/AP_Notify/ToneAlarm_Linux.cpp

@@ -37,7 +37,7 @@ bool ToneAlarm_Linux::init()
 {
     // open the tone alarm device
     err = hal.util->toneAlarm_init();
-    if (err == -1) {
+    if (err) {
         hal.console->printf("AP_Notify: Failed to initialise ToneAlarm");
         return false;
     }

libraries/AP_Notify/ToneAlarm_Linux.h

@@ -36,7 +36,7 @@ private:
     /// play_tune - play one of the pre-defined tunes
     bool play_tune(uint8_t tune_number);
 
-    int err;
+    bool err;
 
     /// tonealarm_type - bitmask of states we track
     struct tonealarm_type {