Bluetooth: AMP: Get amp_mgr reference in HS hci_conn

When assigning amp_mgr in hci_conn (type AMP_LINK) get also reference.
In hci_conn_del those references would be put for both conn types
AMP_LINK and ACL_LINK associated with amp_mgr.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>