Mac: Fix return value for string functions

get_serial_number()
	get_manufacturer_string()
	get_product_string()

This is to match the documentation in the header file, returning 0 on
success and -1 on error.

mac/hid.c

@@ -291,10 +291,10 @@ static int get_string_property(IOHIDDeviceRef device, CFStringRef prop, wchar_t
 			&used_buf_len);
 
 		buf[chars_copied] = 0;
-		return chars_copied;
+		return 0;
 	}
 	else
-		return 0;
+		return -1;
 		
 }