Bluetooth: Fix Set Extended (Scan Response) Data

These command do have variable length and the length can go up to 251,
so this changes the struct to not use a fixed size and then when
creating the PDU only the actual length of the data send to the
controller.

Fixes: a0fb3726 ("Bluetooth: Use Set ext adv/scan rsp data if controller supports")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>