AP_GPS: make NMEA driver a bit easier to read

thanks to crashpilot100

libraries/AP_GPS/AP_GPS_NMEA.cpp

@@ -94,6 +94,7 @@ const char AP_GPS_NMEA::_gpvtg_string[] PROGMEM = "GPVTG";
 // Convenience macros //////////////////////////////////////////////////////////
 //
 #define DIGIT_TO_VAL(_x)        (_x - '0')
+#define hexdigit(x) ((x)>9?'A'+(x):'0'+(x))
 
 AP_GPS_NMEA::AP_GPS_NMEA(AP_GPS &_gps, AP_GPS::GPS_State &_state, AP_HAL::UARTDriver *_port) :
     AP_GPS_Backend(_gps, _state, _port),
@@ -181,9 +182,9 @@ uint32_t AP_GPS_NMEA::_parse_decimal_100()
         ++p;
     if (*p == '.') {
         if (isdigit(p[1])) {
-            ret += 10 * (p[1] - '0');
+            ret += 10 * DIGIT_TO_VAL(p[1]);
             if (isdigit(p[2]))
-                ret += p[2] - '0';
+                ret += DIGIT_TO_VAL(p[2]);
         }
     }
     return ret;
@@ -223,7 +224,8 @@ uint32_t AP_GPS_NMEA::_parse_degrees()
         q = p + 1;
         float frac_scale = 0.1f;
         while (*q && isdigit(*q)) {
-            frac_min += (*q++ - '0') * frac_scale;
+            frac_min += DIGIT_TO_VAL(*q) * frac_scale;
+            q++;
             frac_scale *= 0.1f;
         }
     }
@@ -375,8 +377,6 @@ bool AP_GPS_NMEA::_term_complete()
     return false;
 }
 
-#define hexdigit(x) ((x)>9?'A'+(x):'0'+(x))
-
 /*
   detect a NMEA GPS. Adds one byte, and returns true if the stream
   matches a NMEA string