Bluetooth: Audio: Shell: adds support for BIS index parameter

When calling bap_broadcast_assistant add_pa_sync, it should only
set the BIS index field as optional parameters and not to whatever
is in the BASE.

If setting BIS index which the BASE does not support, then the
command should be rejected.

This PR fixes https://github.com/zephyrproject-rtos/zephyr/issues/70835



Signed-off-by: Ping Wang <pinw@demant.com>