ofproto-sflow: Maintain table of ports even when clearing configuration.

ofproto-sflow: Maintain table of ports even when clearing configuration.

When ofproto_sflow_set_options() fails, it calls ofproto_sflow_clear() to
deconfigure the ofproto_sflow object.  But ofproto_sflow_clear() deletes
all of the object's record of datapath ports.  That means that the next
call to ofproto_sflow_set_options(), if it succeeds, will believe that the
datapath has no ports.

This commit fixes the problem by only clearing ofproto_sflow's record of
datapath ports when it is destroyed, not just when a configuration error
occurs.

Reported-by: Neil McKee <neil.mckee@inmon.com>