Bluetooth: debug: Fix printing refcnt for hci_conn

Use the same style for refcnt printing through all Bluetooth code
taking the reference the l2cap_chan refcnt printing.

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