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
(cherry picked from commit a431adf8676ea377175b91776082eb37d4773e80)