Bluetooth: AMP: Physical link struct and helpers

Define physical link structures. Physical links are represented by
hci_conn structure. For BR/EDR we use type ACL_LINK and for AMP
we use AMP_LINK.

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