openvswitch: use vport instead of p

All functions used struct vport *vport except
ovs_vport_find_upcall_portid.

This fixes 1 kerneldoc warning

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>