drivers/xbee: disable CTS/RTS pin unless periph_uart_hw_fc is used

If usinng an arduino xbee shield then CTS pin will be wired to the
ICSP connector RESET pin.

If also used on an arduino-% board then if xbee is busy sending or
receiving data it will assert CTS and this will trigger a reset on
the board.

To avoid this disable CTS functionality on start up unless hardware
flow control is explicitly requested.

drivers/xbee/xbee.c

@@ -599,6 +599,14 @@ int xbee_init(netdev_t *dev)
  _at_cmd(xbee, "ATMM2\r");
  /* put XBee module in "API mode without escaped characters" */
  _at_cmd(xbee, "ATAP1\r");
+ /* disable xbee CTS and RTS, unless hardware flow control is used */
+ if(!IS_USED(MODULE_PERIPH_UART_HW_FC)) {
+ DEBUG("[xbee] init: WARNING if using an arduino BOARD + arduino xbee " \
+ "shield with ICSP connector, hardware flow control can't be " \
+ "used since CTS pin is connected to ICSP RESET pin\n");
+ _at_cmd(xbee, "ATD6 0\r");
+ _at_cmd(xbee, "ATD7 0\r");
+ }
  /* apply AT commands */
  _at_cmd(xbee, "ATAC\r");
  /* exit command mode */