LE: Memory leak when caching GATT service change

When a GATT connection is disconnected, a bonded device will be added
into service change pending list. But if the device is already in the
list, a duplicate record will be added and one buffer is lost.

This change ensures that devices are only added if they are not already
in the list.

Change-Id: Ied6872b145ed28bde8044444cb6c9cbee525943d