Negative times are unlikly

While bunting an other bug I stumbled across:
ultralcd.cpp:1250: warning: comparison between signed and unsigned
integer expressions
Changed to the type of lcd_next_update_millis.

Marlin/ultralcd.cpp

@@ -1246,7 +1246,7 @@ void lcd_update() {
     }
   #endif//CARDINSERTED
 
-  long ms = millis();
+  uint32_t ms = millis();
   if (ms > lcd_next_update_millis) {
 
     #ifdef ULTIPANEL