Bluetooth: Controller: Fix BIS max PDU size used in calc of sub_interval

Fix BIS max PDU size used in calc of sub_interval, use the
minimum of supported maximum BIS PDU size and the requested
SDU size. This will avoid wasted air time between subevents
when SDU size is less than maximum supported PDU size, where
only 150 us tIFS is mandatory between subevents.

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