[partition] Also update combo box index when setting up

- Avoids case where you edit a partition with a mountpoint
   set; previously, calling setText() would update the text
   but leave the selected index unchanged (usually 0), so that
   later calling selectedMountPoint() would return empty.

src/modules/partition/gui/MountPoints.cpp

@@ -51,10 +51,7 @@ void
 standardMountPoints(QComboBox& combo, const QString& selected)
 {
     standardMountPoints( combo );
-    if ( selected.isEmpty() )
-        combo.setCurrentIndex( 0 );
-    else
-        combo.setCurrentText( selected );
+    setSelectedMountPoint( combo, selected );
 }
 
 QString