AP_Notify: fix wrong boolean check

Check by false instead of checking by -1. Fix the following compiler warning
with gcc 5.1.0:

ardupilot/libraries/AP_Notify/ToneAlarm_Linux.cpp:64:13: warning: comparison of constant '-1' with boolean expression is always false [-Wbool-compare]
     if (err == -1) {
             ^

Also change the initialization code not to use -1.

libraries/AP_Notify/ToneAlarm_Linux.cpp

@@ -61,7 +61,7 @@ bool ToneAlarm_Linux::play_tune(uint8_t tune_number)
 void ToneAlarm_Linux::update()
 {
     // exit immediately if we haven't initialised successfully
-    if (err == -1) {
+    if (err) {
         return;
     }
 

libraries/AP_Notify/ToneAlarm_Linux.h

@@ -23,9 +23,9 @@
 class ToneAlarm_Linux: public NotifyDevice
 {
 public:
-    ToneAlarm_Linux():
-    err(-1)
-    {}
+    ToneAlarm_Linux() : err(true)
+    { }
+
     /// init - initialised the tone alarm
     bool init(void);