AP_RTC: tighten types on get_system_clock_utc

None of these components should ever be negative - given its source is
only ever seconds-since-epoch.

libraries/AP_RTC/AP_RTC.cpp

@@ -72,7 +72,7 @@ bool AP_RTC::get_utc_usec(uint64_t &usec) const
     return true;
 }
 
-bool AP_RTC::get_system_clock_utc(int32_t &hour, int32_t &min, int32_t &sec, int32_t &ms)
+bool AP_RTC::get_system_clock_utc(uint8_t &hour, uint8_t &min, uint8_t &sec, uint16_t &ms)
 {
      // get time of day in ms
     uint64_t time_ms = 0;
@@ -115,7 +115,8 @@ uint32_t AP_RTC::get_time_utc(int32_t hour, int32_t min, int32_t sec, int32_t ms
     }
 
     // get start_time_ms as h, m, s, ms
-    int32_t curr_hour, curr_min, curr_sec, curr_ms;
+    uint8_t curr_hour, curr_min, curr_sec;
+    uint16_t curr_ms;
     if (!get_system_clock_utc(curr_hour, curr_min, curr_sec, curr_ms)) {
         return 0;
     }

libraries/AP_RTC/AP_RTC.h

@@ -37,7 +37,7 @@ public:
     /*
       get time in UTC hours, minutes, seconds and milliseconds
      */
-    bool get_system_clock_utc(int32_t &hour, int32_t &min, int32_t &sec, int32_t &ms);
+    bool get_system_clock_utc(uint8_t &hour, uint8_t &min, uint8_t &sec, uint16_t &ms);
 
     uint32_t get_time_utc(int32_t hour, int32_t min, int32_t sec, int32_t ms);
 

libraries/AP_RTC/examples/RTC_test/RTC_Test.cpp

@@ -70,7 +70,8 @@ void loop(void)
     }
     { // generally make sure time is moving forward / initial time
         // offset looks right
-        int32_t hour, min, sec, ms;
+        uint8_t hour, min, sec;
+        uint16_t ms;
         if (!rtc.get_system_clock_utc(hour, min, sec, ms)) {
             failed("Failed to get hour/min/sec/ms");
             return;