Bluetooth: use l2cap_chan_set_err()

l2cap_conn_unreliable() doesn't take the sk lock, so we need to take it
using l2cap_chan_set_err().

Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>