Bluetooth: Add error return value to hci_req_sync callback

In some circumstances it may be useful to abort the request through
checks done in the request callback. To make the feature possible this
patch changes the return value of the request callback from void to
int and aborts the request if a non-zero value is returned.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>