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.com


Signed-off-by: Alex Williamson <alex.williamson@redhat.com>