openvswitch: Use sw_flow_key_range for key ranges.

These minor tidyups make a future patch a little tidier.

Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>