selftests/bpf/test_offload.py: Reset ethtool features after failed setting

When setting the ethtool feature flag fails (as expected for the test), the
kernel now tracks that the feature was requested to be 'off' and refuses to
subsequently disable it again. So reset it back to 'on' so a subsequent
disable (that's not supposed to fail) can succeed.

Fixes: 417ec264 ("selftests/bpf: add offload test based on netdevsim")
Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/bpf/160752226280.110217.10696241563705667871.stgit@toke.dk