nrf: ble_uart: Feed the 'dog when transmitting characters

On systems that cannot feed the watchdog from an ISR (e.g. anything
without a button) then tab completion is risky because the watchdog might
fire before all the options are listed. Fix this by feeding the dog from
the tx routine.

Signed-off-by: Daniel Thompson <daniel@redfelineninja.org.uk>

ports/nrf/drivers/bluetooth/ble_uart.c

@@ -117,6 +117,8 @@ int mp_hal_stdin_rx_chr(void) {
 }
 
 void mp_hal_stdout_tx_strn(const char *str, size_t len) {
+    wdt_feed(false);
+
     // Not connected: drop output
     if (!ble_uart_enabled()) return;