The right place to add the debugfs create is in
setup_driver() and remove it in teardown_driver().

Current code adds the debugfs when creating the device but resetting a
device will remove the debugfs subtree and subsequent set_driver will
not be able to create the files since the debugfs pointer is NULL.

Fixes: 29422100 ("vdpa/mlx5: Add debugfs subtree")
Signed-off-by: Eli Cohen <elic@nvidia.com>

v3 -> v4:
Fix error flow in setup_driver()
Message-Id: <20230403114039.11102-1-elic@nvidia.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Acked-by: Jason Wang <jasowang@redhat.com>