Bluetooth: gatt: add checks for db out of sync on read mult requests

This check is present in att_read_rsp(), but att_read_mult_req and
att_read_mult_vl_req do not use it. Add this check to these functions.

This was affecting GATT/SR/GAS/BV-08-C

Signed-off-by: Krzysztof Kopyściński <krzysztof.kopyscinski@codecoup.pl>