Bluetooth: Use new alloc_workqueue()

Update hdev workqueue API usage to use the new interface, this new
interface also allow us to mark this workqueue as WQ_HIGHPRI, so now rx
and tx work gets higher priority when running.

Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>