net.c: fix check_return false positive from Coverity

In coap_io_do_epoll(), read() is called after timeout to clear the
file descriptor associated with the firing timer. As this function is
only called for its side effect, the result is ignored. Coverity scan
reports this as check_return event. A cast to void has been added to
document that the result is ignored on purpose.

This fixes CID 1495728.

src/net.c

@@ -1586,7 +1586,7 @@ coap_io_do_epoll(coap_context_t *ctx, struct epoll_event *events, size_t nevents
        */
       uint64_t count;
 
-      read(ctx->eptimerfd, &count, sizeof(count));
+      (void)read(ctx->eptimerfd, &count, sizeof(count));
       /* And process any timed out events */
       coap_ticks(&now);
       coap_io_prepare_epoll(ctx, now);