Bluetooth: A2MP: Do not reference hci_conn

Make A2MP channel special channel which do not reference hci_conn.
This prevents from keeping ACL connection open when all L2CAP
channels got closed.

hci_conn_hold and hci_conn_put are not reference counts on the
hci_conn structure in the typical way.  They are reference counts for
the ACL.  When you do the last hci_conn_put, the ACL is disconnected
after a timeout.

Reported-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>