samples: Bluetooth: BAP unicast server fix RX stack size

The sample is currently decoding in the BT RX thread,
and not the system workqueue thread,
which then requires additional stack size.

Ideally it would decode in a separate thread to avoid
expensive operations in the BT RX thread, but that is
outside the scope of this commit.

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