AP_GPS: Improved accuracy of NMEA driver

The conversion of ret (32bit-integer) to float reduced accuracy to ~9cm or ~22cm. Now it's ~1cm.

libraries/AP_GPS/AP_GPS_NMEA.cpp

@@ -231,7 +231,7 @@ uint32_t AP_GPS_NMEA::_parse_degrees()
     }
     ret = (deg * (int32_t)10000000UL);
     ret += (min * (int32_t)10000000UL / 60);
-    ret += frac_min * (1.0e7 / 60.0f);
+    ret += (int32_t) (frac_min * (1.0e7 / 60.0f));
     return ret;
 }