[Bluetooth] Add HCIUARTGETDEVICE support for HCI line discipline

Adding HCIUARTGETDEVICE makes it possible to get the HCI device number
that is attached to a given serial device. This is required during the
initialization process of some Bluetooth chips.

Signed-off-by: Ohad Ben-Cohen <ohad@bencohen.org>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>