datapath: avoid hard coding OVS_VPORT_TYPE_GENEVE

OVS_VPORT_TYPE_GENEVE is currently hard coded to 6. This is not
necessary since slot 5 has not been taken yet. Drop the hard
coded value to before upstreaming GENEVE support to Linux kernel.

Signed-off-by: Andy Zhou <azhou@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>