netdevsim: support ethtool ring and coalesce settings

Add ethtool ring and coalesce settings support for testing.

Signed-off-by: Antonio Cardace <acardace@redhat.com>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>