bmi160_main:Make the internal interface conditional on PX4_SPIDEV_BMI

px4fmuv2 had PX4_SPIDEV_BMI defined, for the v3 cmake, but never
   provided a Chip select decoded by PX4_SPIDEV_BMI. PX4_SPIDEV_BMI
   has been removed from V2, but PX4_SPIDEV_EXT_BMI still remains
   and has a chip select assigned to it.

src/drivers/bmi160/bmi160_main.cpp

@@ -54,7 +54,11 @@ start(bool external_bus, enum Rotation rotation)
 #endif
 
 	} else {
+#if defined(PX4_SPIDEV_BMI)
 		*g_dev_ptr = new BMI160(PX4_SPI_BUS_SENSORS, path_accel, path_gyro, (spi_dev_e)PX4_SPIDEV_BMI, rotation);
+#else
+		errx(0, "No Internal SPI CS");
+#endif
 	}
 
 	if (*g_dev_ptr == nullptr) {