ethtool: don't call set_channels in drivers if config didn't change

Don't call drivers if nothing changed. Netlink code already
contains this logic.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>