atten-pps3xxx: Make serial write call block in scan.

This is called at scan time so free to block. There is no partial write
handling and a response is expected afterwards. This should therefore be a
blocking call.

src/hardware/atten-pps3xxx/api.c

@@ -130,7 +130,7 @@ static GSList *scan(GSList *options, int modelid)
 	memset(packet, 0, PACKET_SIZE);
 	packet[0] = 0xaa;
 	packet[1] = 0xaa;
-	if (serial_write(serial, packet, PACKET_SIZE) == -1) {
+	if (serial_write_blocking(serial, packet, PACKET_SIZE) == -1) {
 		sr_err("Unable to write while probing for hardware: %s",
 				strerror(errno));
 		return NULL;