agilent-dmm: Make serial write call block in scan.

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

src/hardware/agilent-dmm/api.c

@@ -116,7 +116,7 @@ static GSList *scan(GSList *options)
 		return NULL;
 
 	serial_flush(serial);
-	if (serial_write(serial, "*IDN?\r\n", 7) == -1) {
+	if (serial_write_blocking(serial, "*IDN?\r\n", 7) == -1) {
 		sr_err("Unable to send identification string: %s.",
 		       strerror(errno));
 		return NULL;