Bluetooth: Fix set value command handler

If the attribute value is going to be changed by the upper tester,
(eg. to issue notification) for simplicity it can be stored in the
place of old one.

Change-Id: I03a6ebbda5d75e6ffa9c2fd6b3c0eab495623a11
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>