arm_mpam: Fix a NULL pointer dereference on unbinding after an error interrupt

If a user unbinds an MSC after mpam_disable() has been run in response
to an error interrupt then a dereference of a NULL pointer occurs as
mpam_disable() sets the drvdata to NULL. Add an early return to the driver
remove callback to avoid this.

Fixes: f04046f257 ("arm_mpam: Add probe/remove for mpam msc driver and kbuild boiler plate")
Signed-off-by: Ben Horgan <ben.horgan@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>

drivers/resctrl/mpam_devices.c

@@ -2017,6 +2017,9 @@ static void mpam_msc_drv_remove(struct platform_device *pdev)
 {
 	struct mpam_msc *msc = platform_get_drvdata(pdev);
 
+	if (!msc)
+		return;
+
 	mutex_lock(&mpam_list_lock);
 	mpam_msc_destroy(msc);
 	mutex_unlock(&mpam_list_lock);