Bluetooth: Use kref for l2cap channel reference counting

This patch changes the struct l2cap_chan and associated code to use
kref api for object refcounting and freeing.

Suggested-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Jaganath Kanakkassery <jaganath.k@samsung.com>
Signed-off-by: Syam Sidhardhan <s.syam@samsung.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>