Bluetooth: GATT: set subscribe value to zero for unsubscription

Subscribe value is set to zero before calling notify callback,
to distinguish a subscription failure from an unsubscription
on disconnection (when flag BT_GATT_SUBSCRIBE_FLAG_VOLATILE is set).

Change-Id: Ia91220492d82041b2c385bf88a15180387e7a483
Signed-off-by: Jonathan Gelie <jonathanx.gelie@intel.com>