Bluetooth: Rename hci_request to hci_req_sync

We'll be introducing an async version of hci_request. To make things
clear it makes sense to rename the existing API to have a _sync suffix.

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>