QDateTimeEdit: use correct size hint when calendarPopup is enabled

When calendarPopup is enabled, a QComboBox instead a QSpinBox is drawn
but the size hint for QSpinBox was used which resulted in a too small
width for the widget (depending on the underlying style). Therefore use
the size hint for CT_ComboBox in this case as it was already done for
mac.

Fixes: QTBUG-11967
Change-Id: I817bdc99ae5d1189b9f0451a857022abe64b3bf8
Reviewed-by: Volker Hilsheimer <volker.hilsheimer@qt.io>
(cherry picked from commit 0a1c171)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/widgets/qdatetimeedit.cpp

@@ -1092,14 +1092,12 @@ QSize QDateTimeEdit::sizeHint() const
         w += 2; // cursor blinking space
 
         QSize hint(w, h);
-
-#ifdef Q_OS_MAC
         if (d->calendarPopupEnabled()) {
             QStyleOptionComboBox opt;
+            opt.initFrom(this);
+            opt.frame = d->frame;
             d->cachedSizeHint = style()->sizeFromContents(QStyle::CT_ComboBox, &opt, hint, this);
-        } else
-#endif
-        {
+        } else {
             QStyleOptionSpinBox opt;
             initStyleOption(&opt);
             d->cachedSizeHint = style()->sizeFromContents(QStyle::CT_SpinBox, &opt, hint, this);