netdevsim: move ethtool pause params in separate struct

This will help the refactoring in the next commit
when coalesce and ring settings are added.

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