openvswitch: Fix key serialization.

Fix typo where mask is used rather than key.

Fixes: 74ed7ab9

("openvswitch: Add support for unique flow IDs.")
Reported-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>