fw/b: Prevent trying to enable hw offload for tethering via VPN upstreams

* Tethering via VPN upstream requires a sw path.

* hw offload setup happened to be being disabled anyway owing to a fail
  return code from setDataLimit().  However, it was causing offload to be
  disabled entirely (until next hotspot off / on event).

* Gracefully skip hw offload for vpn upstreams so that it is automatically
  used again when a vpn is disconnected.

Change-Id: I4df13f02889305560903b7b1e919eedc7af78c07