Bluetooth: Move l2cap_chan_hold/put to l2cap_core.c

Refactor the code in order to use the l2cap_chan_destroy()
from l2cap_chan_put() under the refcnt protection.

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>