Bluetooth: avoid silent hci_bcm ACPI PM regression

The hci_bcm platform-device hack which was used to implement
power management for ACPI devices is being replaced by a
serial-device-bus implementation.

Unfortunately, when the corresponding change to the ACPI code lands (a
change that will stop enumerating and registering the serial-device-node
child as a platform device) PM will break silently unless serdev
TTY-port controller support has been enabled. Specifically, hciattach
(btattach) would still succeed, but power management would no longer
work.

Although this is strictly a runtime dependency, let's make the driver
depend on SERIAL_DEV_CTRL_TTYPORT, which is the particular serdev
controller implementation used by the ACPI devices currently managed by
this driver, to avoid breaking PM without anyone noticing.

Note that the driver already has a (build-time) dependency on the serdev
bus code.

Signed-off-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>