Bluetooth: Improve debugging messages for hci_conn

Improve debugging of hci_conn objects by: adding print to hci_conn
refcounting, adding object spcifier when missing, change conn to hcon
since conn is heavily used for l2cap_conn objects and this is misleading.

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