ofproto: Enqueue improperly resets priority.

ofproto: Enqueue improperly resets priority.

Before this patch, enqueue would reset the priority of a flow to
its original value instead of the value it had immediately before
the enqueue action.

Thus, these openflow actions:
set_queue:2,enqueue:1:1,output:1

Would get translated into these incorrect datapath actions:
set_priority(0x10003),set_priority(0x10002),1,pop_priority,1

Instead of these correct datapath actions:
set_priority(0x10003),set_priority(0x10002),1,set_priority(0x10003),1

A future patch will remove the redundant "set_priority(0x10003)".

Found by inspection.