Make the string parameters const for do_flow_mod__()

Make the string parameters const for do_flow_mod__()

Make the string parameter of parse_ofp_flow_stats_request_str() and
parse_ofp_flow_mod_str() const

* Both parse_ofp_flow_stats_request_str() and parse_ofp_flow_mod_str()
  only pass their string parameter to it to parse_ofp_str() which treats
  the parameter as const.
* do_flow_mod__() may pass a const string as the string argument to
  parse_ofp_flow_stats_request_str() and parse_ofp_flow_mod_str()

Signed-off-by: Simon Horman <horms@verge.net.au>
Signed-off-by: Ben Pfaff <blp@nicira.com>