Bluetooth: Use req_workqueue for hci_request operations

This patch converts work assignment relying on hci_request() from the
system-global work queue to the per-HCI device specific work queue
(hdev->req_workqueue) intended for hci_request() related tasks.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>