vdpa/mlx5: Fix config_attr_mask assignment

Fix VDPA_ATTR_DEV_NET_CFG_MACADDR assignment to be explicit 64 bit
assignment.

No issue was seen since the value is well below 64 bit max value.
Nevertheless it needs to be fixed.

Fixes: a007d940

 ("vdpa/mlx5: Support configuration of MAC")
Reviewed-by: Si-Wei Liu <si-wei.liu@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Signed-off-by: Eli Cohen <elic@nvidia.com>
Link: https://lore.kernel.org/r/20220105114646.577224-7-elic@nvidia.com
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>