Only add ETHTOOL_OPTIONS when offloading is disabled

Otherwise use the defaults. This is to workaround
https://bugzilla.suse.com/show_bug.cgi?id=900112
Which causes wickedd to fail to bring up virtio interfaces on SLES12 when tx/rx
options are present in ETHTOOL_OPTIONS.
Merge pull request #305 from rhafer/nic-offload

Only add ETHTOOL_OPTIONS when offloading is disabled