vyos.ethtool: T3645: fix compatibility with latest ethtool version
vyos.ethtool: T3645: fix compatibility with latest ethtool version

Ethtool version used on Debian Bullseye changed the output format from 0 -> n/a.
As we are only interested in the tx/rx keys we do not care about RX Mini/Jumbo.

vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x,vyos/vyos-1x