tests: Bluetooth: Tester: Do not send MICP Mute ev on write cb

When the BT Tester gets the write response, it should not send
the state change event, as the write response may come
before the notifications, and in which case the state may
not have changed.

The BT Tester should always wait for the state change notification
before sending the event.

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