openvswitch: Use TCP flags in the flow key for stats.

We already extract the TCP flags for the key, might as well use that
for stats.

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