Bluetooth: BAP: Broadcast sink: Fix bis_sync in update_recv_state_base

In the update_recv_state_base function, we should update the
BASE with any new metadata that we have received (as that may
change while streaming). However, this function did not properly
keep the existing bis_sync values for the
subgroups, causing the bis_sync for all subgroups to be 0.

Signed-off-by: Emil Gydesen <emil.gydesen@nordicsemi.no>