ovs-ofctl: Use the prepared connection to dump flows in do_dump_flows__().

ovs-ofctl: Use the prepared connection to dump flows in do_dump_flows__().

The logic in do_dump_flows__() went to some trouble to open an OpenFlow
connection and set the correct protocol, but then it allowed
dump_stats_transaction() to create and use a completely different OpenFlow
connection that hadn't been prepared that way.  This commit fixes the
problem.

I don't think that there is a real bug here because currently the set of
protocols doesn't influence flow stats replies.  But that could change in
the future.

Signed-off-by: Ben Pfaff <blp@nicira.com>