openvswitch: Validation of IPv6 set port action uses IPv4 header

When the kernel validates set TCP/UDP port actions, it looks at
the ports in the existing flow to make sure that the L4 header exists.
However, these actions always use the IPv4 version of the struct.
Following patch fixes this by checking for flow ip protocol first.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: Jesse Gross <jesse@nicira.com>