Bluetooth: Remove unneeded hci_conn_disconnect abstraction

The abstraction of disconnect operation via hci_conn_disconnect is not
needed and it does not add any readability. Handle the difference of
AMP physical channels and BR/EDR/LE connection in the timeout callback.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>