platform/x86: ISST: Fix wrong unregister type

The MMIO driver is not unregistering with the correct type with the ISST
common core during module removal. This should be unregistered with
ISST_IF_DEV_MMIO instead of ISST_IF_DEV_MBOX.
Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>

drivers/platform/x86/intel_speed_select_if/isst_if_mmio.c

@@ -126,7 +126,7 @@ static void isst_if_remove(struct pci_dev *pdev)
 	struct isst_if_device *punit_dev;
 
 	punit_dev = pci_get_drvdata(pdev);
-	isst_if_cdev_unregister(ISST_IF_DEV_MBOX);
+	isst_if_cdev_unregister(ISST_IF_DEV_MMIO);
 	mutex_destroy(&punit_dev->mutex);
 }