Bluetooth: host: Ignore return value of change-aware when reading DBhash

Ignore the return value of the bt_gatt_change_aware function when the
client is reading the database hash characteristic value. This is the
point where the client becomes change-aware, so nothing else should be
done if the client is change-unaware.

Fixes: #38012

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>