Bluetooth: Audio: Add value_handle check to MCC subscribe cb

If the subscribed callback is called with value_handle = 0,
then it should not be treated as a subscription complete.

This will happen when bt_mcc_discover_mcs is called, as
reset_mcs_inst unsubscribed to all characteristics before
discovering again.

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