ArduCopter: fix signed/unsigned comparison warning

commands_logic.cpp: In member function 'bool
Copter::verify_within_distance()':
commands_logic.cpp:770:21: warning: comparison between signed and
unsigned integer expressions [-Wsign-compare]
     if (wp_distance < MAX(condition_value,0)) {
	                          ^

ArduCopter/commands_logic.cpp

@@ -767,7 +767,7 @@ bool Copter::verify_within_distance()
 {
     // update distance calculation
     calc_wp_distance();
-    if (wp_distance < MAX(condition_value,0)) {
+    if (wp_distance < (uint32_t)MAX(condition_value,0)) {
         condition_value = 0;
         return true;
     }