Bluetooth: Remove reason parameter from hci_amp_disconn function

The hci_amp_disconn function is a local function and there is no
need for a reason parameter. That one can be retrieved from the
hci_conn object easily.

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