Bluetooth: Check req->err in hci_req_add

If req->err is set, there is no point in queueing the HCI command
in HCI request command queue since it won't be sent anyway.

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>