bluetooth: gatt: Fix ATT Read By Type by DB change unaware client

When change unaware client send ATT request it shall get Database
Out of Sync error. Reading GATT database hash is an exception here.

This was affecting GATT/SR/GAS/BV-05-C qualification test case.

Signed-off-by: Szymon Janc <szymon.janc@codecoup.pl>