px4fmu-v5: make sure the internal ist8310 is detected as internal mag

ROMFS/px4fmu_common/init.d/rc.sensors

@@ -285,7 +285,7 @@ then
 	ist8310 -C -b 2 start
 
 	# Possible internal compass
-	ist8310 -C -b 3 start
+	ist8310 -C -b 5 start
 fi
 
 if ver hwcmp AEROCORE2

src/drivers/boards/px4fmu-v5/board_config.h

@@ -215,6 +215,7 @@
 #define PX4_I2C_BUS_EXPANSION2	3
 #define PX4_I2C_BUS_EXPANSION3	4
 #define PX4_I2C_BUS_LED			PX4_I2C_BUS_EXPANSION
+#define PX4_I2C_BUS_ONBOARD	PX4_I2C_BUS_EXPANSION2
 
 #define BOARD_NUMBER_I2C_BUSES  4
 #define BOARD_I2C_BUS_CLOCK_INIT {100000, 100000, 100000, 100000}