comdlg32: Use proper helpers for iface calls

[Marvin]

Hi,

While running your changed tests on Windows, I think I found new failures.
Being a bot and all I'm not very good at pattern recognition, so I might be
wrong, but could you please double-check?
Full results can be found at
http://testbot.winehq.org/JobDetails.pl?Key=20929

Your paranoid android.


=== TEST64_W7SP1 (32 bit itemdlg) ===
itemdlg: unhandled exception c0000005 at 75AF1AC3