fmu-v5: remove i2c3 as bus expansion because used just for internal mag (ONBOARD)

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

@@ -228,10 +228,9 @@
 
 #define PX4_I2C_BUS_EXPANSION	1
 #define PX4_I2C_BUS_EXPANSION1	2
-#define PX4_I2C_BUS_EXPANSION2	3
-#define PX4_I2C_BUS_EXPANSION3	4
+#define PX4_I2C_BUS_ONBOARD		3
+#define PX4_I2C_BUS_EXPANSION2	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}