Prevent too tight loop on the 1st core

src/main.c

@@ -598,6 +598,8 @@ static void do_rx()
 		if (queue_try_remove(&iq_queue, &block)) {
 			fwrite(block, IQ_BLOCK_LEN, 1, stdout);
 			fflush(stdout);
+		} else {
+			sleep_us(25);
 		}
 	}