Bluetooth: hci_uart_set_flow_control: Fix NULL deref when using serdev

Fix a NULL pointer deref (hu->tty) when calling hci_uart_set_flow_control
on hci_uart-s using serdev.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>