netinfo: fix getMaxMtu generator handling

getMaxMtu was changed to use iterables instead of just lists.
The check for if and a generator is always True regardless of
it generating any elements. Thus, max could fail.

This fixes a regression introduced by:
http://gerrit.ovirt.org/15355

Change-Id: I94a58ab6a3303099105987fd230680586f6a3cce
Signed-off-by: Antoni S. Puimedon <asegurap@redhat.com>
Reviewed-on: http://gerrit.ovirt.org/15903
Reviewed-by: Giuseppe Vallarelli <gvallare@redhat.com>
Reviewed-by: Dan Kenigsberg <danken@redhat.com>
Tested-by: Dan Kenigsberg <danken@redhat.com>