AP_Compass: disable lis3mdl for now

pending investigation of possible hw error

libraries/AP_Compass/AP_Compass.cpp

@@ -517,13 +517,15 @@ void Compass::_detect_backends(void)
                                               both_i2c_external, both_i2c_external?ROTATION_ROLL_180:ROTATION_YAW_270),
                     AP_Compass_HMC5843::name, both_i2c_external);
 
-        // lis3mdl
+#if 0
+        // lis3mdl - this is disabled for now due to an errata on pixhawk2 GPS unit, pending investigation
         ADD_BACKEND(AP_Compass_LIS3MDL::probe(*this, hal.i2c_mgr->get_device(1, HAL_COMPASS_LIS3MDL_I2C_ADDR),
                                                true, ROTATION_YAW_90),
                      AP_Compass_LIS3MDL::name, true);
         ADD_BACKEND(AP_Compass_LIS3MDL::probe(*this, hal.i2c_mgr->get_device(0, HAL_COMPASS_LIS3MDL_I2C_ADDR),
                                               both_i2c_external, both_i2c_external?ROTATION_YAW_90:ROTATION_NONE),
                      AP_Compass_LIS3MDL::name, both_i2c_external);
+#endif
 
         // AK09916
         ADD_BACKEND(AP_Compass_AK09916::probe(*this, hal.i2c_mgr->get_device(1, HAL_COMPASS_AK09916_I2C_ADDR),