Bluetooth: Make hci_req_add returning void

Since no one checks the returning value of hci_req_add and HCI
request errors are now handled in hci_req_run, we can make hci_
req_add returning void.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>