Bluetooth: BAP: Fix BIS_Sync value in update_recv_state_base

The BASE doesn't affect the BIS sync value, and should thus not
change it.

This fixes a bug where the broadcast sink may prematurely
set the BIS sync value to e.g. 1 before actually
syncing to the BIG.

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