smoketest: ethernet: bugfix RPS CPU validation
smoketest: ethernet: bugfix RPS CPU validation

We always read back the RPS value from eth1 which worked on the CI smoketests,
but not when running a smoketest with only one interface e.g. by:

$ TEST_ETH="eth2" /usr/libexec/vyos/tests/smoke/cli/test_interfaces_ethernet.py

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