openvswitch: Fix flow mask validation.

Following patch fixes typo in the flow validation. This prevented
installation of ARP and IPv6 flows.

Fixes: 19e7a3df

 ("openvswitch: Fix NDP flow mask validation")
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Reviewed-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>