ovs_stats plugin: fix some more build warnings

ovs_stats plugin: fix some more build warnings

CC       src/ovs_stats_la-ovs_stats.lo
src/ovs_stats.c:158:8: warning: 'const' type qualifier on return type has no effect [-Wignored-qualifiers]
static const iface_counter ovs_stats_counter_name_to_type(const char *counter) {
       ^~~~~~
src/ovs_stats.c:356:29: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
          for (int i = 0; i < ports_num; i++)
                          ~ ^ ~~~~~~~~~
src/ovs_stats.c:401:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_OBJECT(bridges)->len; i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ovs_stats.c:415:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_OBJECT(bridges)->len; i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ovs_stats.c:501:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_OBJECT(ports)->len; i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ovs_stats.c:524:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_OBJECT(ports)->len; i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ovs_stats.c:538:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_ARRAY(stats)->len; i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ovs_stats.c:558:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_ARRAY(ext_ids)->len; i++) {
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
src/ovs_stats.c:684:23: warning: comparison of integers of different signs: 'int' and 'size_t' (aka 'unsigned long') [-Wsign-compare]
    for (int i = 0; i < YAJL_GET_OBJECT(ports)->len; i++)
                    ~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~~
9 warnings generated.