Bluetooth: BAP: Fix issue with sink recv state notifications

There was a bug in pa_decode_base that would would spent time
parsing incoming BASEs and also update the receive
states, which caused some tests to fail.

This commit adds a simply check to verify that the BASE is
different before spending parsing the content and updating
the receive states.

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