Fix typo where mask is used rather than key.

Fixes: 74ed7ab9("openvswitch: Add support for unique flow IDs.")
Reported-by: NJoe Stringer <joestringer@nicira.com>
Signed-off-by: NPravin B Shelar <pshelar@nicira.com>
Acked-by: NJoe Stringer <joestringer@nicira.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>