drm/i915/gvt: return error when failing to take the module reference

When we fail to take the module reference, we go to the 'undo*' branch and
return. But the returned variable 'ret' has been set as zero by the
above code. Change 'ret' to '-ENODEV' in this situation.

Fixes: 9bdb073464d6 ("drm/i915/gvt: Change KVMGT as self load module")
Reviewed-by: Zhenyu Wang <[e-mail]>
Reported-by: Hulk Robot <[e-mail]>
Signed-off-by: Xiongfeng Wang <[e-mail]>
Signed-off-by: Zhenyu Wang <[e-mail]>
Link: http://patchwork.freedesktop.org/patch/msgid/[e-mail]

drivers/gpu/drm/i915/gvt/kvmgt.c

···
 	/* Take a module reference as mdev core doesn't take
 	 * a reference for vendor driver.
 	 */
-	if (!try_module_get(THIS_MODULE))
+	if (!try_module_get(THIS_MODULE)) {
+		ret = -ENODEV;
 		goto undo_group;
+	}
 
 	ret = kvmgt_guest_init(mdev);
 	if (ret)