Bluetooth: att: Fix invalid discovered descriptors parsing

This patch fixes the issue with 128bit uuid descriptors discovery.
Data from Find Information Response were parsed improperly,
because length took into account the size of pointer to info data,
not data itself.

Change-Id: Ifad0110705bacc3c757a91ebbd97af5ba93897d9
Signed-off-by: Mariusz Skamra <mariusz.skamra@tieto.com>