qeth: Move away from using neighbour entries in qeth_l3_fill_header()

We've moving to a model where dst_entry objects to not have
a reference to the associated neighbour entry, instead such
neighbours must be looked up on-demand.

Here in qeth_l3_fill_header() it's actually much simpler to
use the information in the route itself.  The code is
already conditionalized upon protocol type.

Signed-off-by: David S. Miller <davem@davemloft.net>
Tested-by: Ursula Braun <ubraun@linux.vnet.ibm.com>