datapath: Fix memory leak in ovs_vport_alloc() error path

ovs_vport_alloc() bails out without freeing the memory 'vport' points
to.

Picked up by Coverity - CID 1230503.

Fixes: beb1c69a3 ("datapath: Allow each vport to have an array of
'port_id's.")
Signed-off-by: Christoph Jaeger <cj@linux.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>