Bluetooth: Controller: Fix BIG Create silently fail on RTN/SDU_interval

Fix BIG Create implementation from silently failing on
values of RTN and SDU_interval that lead to BIG events that
cannot be acheived when using sequential or interleaved
subevents by the Controller Implementation.

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