MPU6000: Use signed addition when accumulating readings.

- Fixes very erratic accel readings on the PX4 board.

libraries/AP_InertialSensor/AP_InertialSensor_MPU6000.cpp

@@ -376,7 +376,7 @@ void AP_InertialSensor_MPU6000::_read_data_transaction() {
     _spi->transaction(tx, rx, 15);
 
     for (uint8_t i = 0; i < 7; i++) {
-        _sum[i] += ((rx[2*i+1] << 8) | rx[2*i+2]);
+        _sum[i] += (int16_t)(((uint16_t)rx[2*i+1] << 8) | rx[2*i+2]);
     }   
     
     _count++;