hmc5883: Fix for Issue1858 detection of MAG on Int/Ext I2C Bus.

src/drivers/hmc5883/hmc5883_i2c.cpp

@@ -113,11 +113,17 @@ HMC5883_I2C::ioctl(unsigned operation, unsigned &arg)
 	switch (operation) {
 
 	case MAGIOCGEXTERNAL:
+// On PX4v1 the MAG can be on an internal I2C
+// On everything else its always external
+#ifdef CONFIG_ARCH_BOARD_PX4FMU_V1
 		if (_bus == PX4_I2C_BUS_EXPANSION) {
 			return 1;
 		} else {
 			return 0;
 		}
+#else
+                return 1;
+#endif
 
 	case DEVIOCGDEVICEID:
 		return CDev::ioctl(nullptr, operation, arg);