vfio/mlx5: Fix to not use 0 as NULL pointer

Fix sparse warning to not use plain integer (i.e. 0) as NULL pointer.
Reported-by: kernel test robot <lkp@intel.com>
Fixes: 6fadb021 ("vfio/mlx5: Implement vfio_pci driver for mlx5 devices")
Signed-off-by: Yishai Hadas <yishaih@nvidia.com>
Link: https://lore.kernel.org/r/202203090703.kxvZumJg-lkp@intel.com
Link: https://lore.kernel.org/r/20220309080217.94274-1-yishaih@nvidia.comSigned-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/pci/mlx5/main.c

@@ -409,7 +409,7 @@ mlx5vf_pci_step_device_state_locked(struct mlx5vf_pci_core_device *mvdev,
 
 	if ((cur == VFIO_DEVICE_STATE_STOP_COPY && new == VFIO_DEVICE_STATE_STOP)) {
 		mlx5vf_disable_fds(mvdev);
-		return 0;
+		return NULL;
 	}
 
 	if (cur == VFIO_DEVICE_STATE_STOP && new == VFIO_DEVICE_STATE_RESUMING) {
@@ -430,7 +430,7 @@ mlx5vf_pci_step_device_state_locked(struct mlx5vf_pci_core_device *mvdev,
 		if (ret)
 			return ERR_PTR(ret);
 		mlx5vf_disable_fds(mvdev);
-		return 0;
+		return NULL;
 	}
 
 	/*