Fix comparison.

Change-Id: Id90a49e57d25d12936678fecfa9c4e0c6dbe676b
Reviewed-on: http://codereview.qt.nokia.com/3255
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/gui/kernel/qsurfaceformat.cpp

@@ -337,7 +337,7 @@ void QSurfaceFormat::setGreenBufferSize(int size)
 
 void QSurfaceFormat::setBlueBufferSize(int size)
 {
-    if (d->blueBufferSize = size) {
+    if (d->blueBufferSize != size) {
         detach();
         d->blueBufferSize = size;
     }