dpdkevents plugin: fix sign compare warnings

src/dpdkevents.c: In function ‘dpdk_helper_link_status_get’:
src/dpdkevents.c:431:21: warning: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Wsign-compare]
   for (int i = 0; i < ec->nb_ports; i++) {
                     ^
src/dpdkevents.c: In function ‘dpdk_events_link_status_dispatch’:
src/dpdkevents.c:500:21: warning: comparison of integer expressions of different signedness: ‘int’ and ‘uint32_t’ {aka ‘unsigned int’} [-Wsign-compare]
   for (int i = 0; i < ec->nb_ports; i++) {
                     ^