Bluetooth: Add return parameter to cmd_complete callbacks

The cmd_complete callbacks for pending mgmt commands may fail e.g. in
the case of memory allocation. Previously this error would be caught and
returned to user space in the form of a failed write on the mgmt socket
(when the error happened in the mgmt command handler) but with the
introduction of the generic cmd_complete callback this information was
lost. This patch returns the feature by making cmd_complete callbacks
return int instead of void.

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