ifcfg: always write ethernet.s390-options even without subchannels

For the umpteenth time: it is not ifcfg-rh writers decision to decide
what are valid configurations and only persist settings based on
some other settings.

If s390-options would only be allowed together with subchannels, then
this is alone nm_connection_verify()'s task to ensure.

Reproduce with

  $ nmcli connection add type ethernet autoconnect no con-name zz ethernet.s390-options bridge_role=primary

Related: https://bugzilla.redhat.com/show_bug.cgi?id=1935842

Fixes: 16bccfd6 ('core: handle s390 options more cleanly')
(cherry picked from commit d391f207)