Bluetooth: Refactor l2cap_send_disconn_req

l2cap_send_disconn_req takes 3 parameters of which conn might be
derived from chan. Make this conversion inside l2cap_send_disconn_req.

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