Bluetooth: Free the l2cap channel list only when refcount is zero

Move the l2cap channel list chan->global_l under the refcnt
protection and free it based on the refcnt.

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