xtensa: ISS: add comment about etherdev freeing

iss_net_configure explicitly frees etherdev in all error return paths
except one where register_netdevice fails. In that remaining error
return path the etherdev is freed by the iss_net_pdev_release callback
triggered by the platform_device_unregister call. Add a comment stating
that.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>