Bluetooth: Controller: Use conn interval unit for Auxiliary event

Use connection interval units of 1250 us for periodic
scheduling of Extended Advertising auxiliary PDU events so
that auxiliary events can be periodically grouped alongwith
the Periodic Advertising Events. This will permit
mitigating overlaps amongst them.

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