Bluetooth: controller: Fixup Broadcast ISO pre-transmission groups > 1

Fixup payload_index to be uint16_t to avoid index overflow.

Do not remember why ptc is 4 bits, where as it must be 5 bit
value similar to nse; added an assertion check until it is
fixed.

Fix ISO Broadcaster and ISO Sync Receiver for PTO > 1 and
use of Pre-Transmission Group Counts.

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