Bluetooth: Fix L2CAP locking scheme regression

When locking was introduced the error path branch was not taken
into account. Error was found in sparse code checking. Kudos to
Jani Nikula.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@nokia.com>
Acked-by: Gustavo F. Padovan <gustavo@las.ic.unicamp.br>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>