main: Use unsigned modulo

Explicitly affix the literal with `U` so SDCC will directly call
`moduint()` instead of `modsint()`.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/board/system76/common/main.c

@@ -99,7 +99,7 @@ void main(void) {
     uint32_t last_time_fan = 0;
 
     for(main_cycle = 0; ; main_cycle++) {
-        switch (main_cycle % 3) {
+        switch (main_cycle % 3U) {
             case 0:
                 // Handle power states
                 power_event();