Fix finding menu items in wxMenuItem::DoSetBitmap.

FindChildItem gets passed the id of a menu item as returned by GetMSWId, however GetMSWId should be used in combination with Win32 API functions. Instead use GetId: this fixes finding menu items that have a negative id (as a result of using wxID_ANY as id) or have a sub menu.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76742 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775