(#18054) Do nothing when trunking encapsulation is negotiate

Given a Cisco WS-C3560-48PS switch with an interface configured in access mode and trunk encapsulation is “negogiate”, when puppet device tries to parse the result of sh interface FastEthernet0/1 switchport, then an exception is raised.

It should not take care of the trunk encapsulation.

See https://projects.puppetlabs.com/issues/18054