drm/vboxvideo: fix a NULL vs IS_ERR() check

The devm_gen_pool_create() function never returns NULL, it returns
error pointers.

Fixes: 4cc9b56545 ("drm/vboxvideo: Use devm_gen_pool_create")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20211118111233.GA1147@kili

drivers/gpu/drm/vboxvideo/vbox_main.c

@@ -127,8 +127,8 @@ int vbox_hw_init(struct vbox_private *vbox)
 	/* Create guest-heap mem-pool use 2^4 = 16 byte chunks */
 	vbox->guest_pool = devm_gen_pool_create(vbox->ddev.dev, 4, -1,
 						"vboxvideo-accel");
-	if (!vbox->guest_pool)
-		return -ENOMEM;
+	if (IS_ERR(vbox->guest_pool))
+		return PTR_ERR(vbox->guest_pool);
 
 	ret = gen_pool_add_virt(vbox->guest_pool,
 				(unsigned long)vbox->guest_heap,