Bluetooth: Call L2CAP teardown callback before clearing chan->conn

L2CAP channel implementations may want to still access the chan->conn
pointer. This will particularly be the case for SMP that will want to
clear a reference to the SMP channel in the l2cap_conn structure. The
only user of the teardown callback so far is l2cap_sock.c and for the
code there it makes no difference whether the callback is called before
or after clearing the chan->conn pointer.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>