It is no longer possible to build BT_HCIUART into the kernel
when SERIAL_DEV_BUS is a loadable module, even if none of the
SERIAL_DEV_BUS based implementations are selected:

drivers/bluetooth/hci_ldisc.o: In function `hci_uart_set_flow_control':
hci_ldisc.c:(.text+0xb40): undefined reference to `serdev_device_set_flow_control'
hci_ldisc.c:(.text+0xb5c): undefined reference to `serdev_device_set_tiocm'

This adds a dependency to avoid the broken configuration.

Fixes: 7841d554 ("Bluetooth: hci_uart_set_flow_control: Fix NULL deref when using serdev")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>