Bluetooth: Rename cmd_complete() to mgmt_cmd_complete()

This patch renames the cmd_complete() function to mgmt_cmd_complete() in
preparation of making it a generic helper for other modules to use too.
Signed-off-by: NJohan Hedberg <johan.hedberg@intel.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>