Bluetooth: Rename create_ad into create_adv_data

Rename the create_ad function into create_adv_data to make it clear
that it is used to create the advertising data. This is important
since later on a function adding the scan response data will be
added.

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