ionic: no devlink_unregister if not registered

ionic: no devlink_unregister if not registered

Don't try to unregister the devlink if it hasn't been registered
yet.  This bit of error cleanup code got missed in the recent
devlink registration changes.

Fixes: 7911c8bd546f ("ionic: Move devlink registration to be last devlink command")
Signed-off-by: Shannon Nelson <snelson@pensando.io>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Link: https://lore.kernel.org/r/20211012231520.72582-1-snelson@pensando.io
Signed-off-by: Jakub Kicinski <kuba@kernel.org>