mshtml: Use helper for IHTMLStyle::put_zIndex implementation.

dlls/mshtml/htmlstyle.c

@@ -2190,22 +2190,7 @@ static HRESULT WINAPI HTMLStyle_put_zIndex(IHTMLStyle *iface, VARIANT v)
 
     TRACE("(%p)->(%s)\n", This, debugstr_variant(&v));
 
-    switch(V_VT(&v)) {
-    case VT_BSTR:
-        return set_style_attr(This, STYLEID_Z_INDEX, V_BSTR(&v), 0);
-    case VT_I4: {
-        WCHAR value[14];
-        static const WCHAR format[] = {'%','d',0};
-
-        wsprintfW(value, format, V_I4(&v));
-        return set_style_attr(This, STYLEID_Z_INDEX, value, 0);
-    }
-    default:
-        FIXME("unimplemented vt %d\n", V_VT(&v));
-        return E_NOTIMPL;
-    }
-
-    return S_OK;
+    return set_nsstyle_attr_var(This->nsstyle, STYLEID_Z_INDEX, &v, 0);
 }
 
 static HRESULT WINAPI HTMLStyle_get_zIndex(IHTMLStyle *iface, VARIANT *p)