openvswitch: Split ovs_flow_cmd_new_or_set().

Following patch will be easier to reason about with separate
ovs_flow_cmd_new() and ovs_flow_cmd_set() functions.

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