Bluetooth: Controller: Initialize BIG info RFU field

The BIG info new Framing Mode field was not initialiazed, so
random garbage was being sent to the air in this field.
For the controller this field is still RFU.
The spec specifies RFU fields must be initialiazed to
0. So let's do so to avoid sending invalid data,
and avoid having random data in the air which can cause
random differences in testcases.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>