Bluetooth: Controller: Fix extended header data be zero-length array

Fix the definiion of Common Extended Payload Format data
field in the PDU definitions to be zero-length array,
because PDU size are configurable and to avoid allocations
being made using these PDU structs.

Corrected the extended scan response length check code to
use the correct define instead.

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>