ComboGrid: fix validation for !allowBlank disabled fields

ComboGrid: fix validation for !allowBlank disabled fields

Used in "Add USB to VM" dialog for example.

This was broken before 15206214d9 "ComboGrid: fix on-load validation for blank
values" (only the one you enabled first was validated, the other always showed
as valid), and afterwards too, but in a different way (both are now immediately
marked invalid until you select and unselect them) - which is how I noticed.

With this the validation now works correctly.

Signed-off-by: Stefan Reiter <s.reiter@proxmox.com>