limit per gpu hashrate logs to 3 sec intervals

may be required for very fast algos, like blake2s

ccminer.cpp

@@ -1712,6 +1712,7 @@ static void *miner_thread(void *userdata)
 
 	while (!abort_flag) {
 		struct timeval tv_start, tv_end, diff;
+		time_t tm_rate_log = 0;
 		unsigned long hashes_done;
 		uint32_t start_nonce;
 		uint32_t scan_time = have_longpoll ? LP_SCANTIME : opt_scantime;
@@ -2302,9 +2303,10 @@ static void *miner_thread(void *userdata)
 			hashlog_remember_scan_range(&work);
 
 		/* output */
-		if (!opt_quiet && loopcnt > 1) {
+		if (!opt_quiet && loopcnt > 1 && (time(NULL) - tm_rate_log) > 3) {
 			format_hashrate(thr_hashrates[thr_id], s);
 			gpulog(LOG_INFO, thr_id, "%s, %s", device_name[dev_id], s);
+			tm_rate_log = time(NULL);
 		}
 
 		/* ignore first loop hashrate */