Bluetooth: GATT: fixing unsubscription

If subcription was found within the subscription list,
we have check if remains identical subscription
from the next node.
Otherwise none unsubscription is realized.

Change-Id: I38132d7c80575801885b8057902f3d4666b08aea
Signed-off-by: Jonathan Gelie <jonathanx.gelie@intel.com>