serial_stream_detect: Make read nonblocking.

This code implements its own waiting based on baudrate, so the read itself
should be nonblocking. In general it will have been already, since drivers
almost universally use the SERIAL_NONBLOCK flag.

src/serial.c

@@ -618,7 +618,7 @@ SR_PRIV int serial_stream_detect(struct sr_serial_dev_inst *serial,
 
 	i = ibuf = len = 0;
 	while (ibuf < maxlen) {
-		len = serial_read(serial, &buf[ibuf], 1);
+		len = serial_read_nonblocking(serial, &buf[ibuf], 1);
 		if (len > 0) {
 			ibuf += len;
 		} else if (len == 0) {