Bluetooth: Fix error handling for l2cap_init()

create_singlethread_workqueue() may fail with errors such as -ENOMEM. If
this happens, the return value is not set to a negative value and the
module load will succeed. It will then crash on module unload because of
a destroy_workqueue() call on a NULL pointer.

Additionally, the _busy_wq workqueue is not being destroyed if any
errors happen on l2cap_init().

Signed-off-by: Anderson Lizardo <anderson.lizardo@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>