main: Replace MOD with bitwise AND

This prevents `moduint` from being called for the superloop.

A bitwise AND is used instead of using the equivalent `% 4` as an
indication that modulo should not be used, as not using a power of 2
(which is optimized) will result in an expensive call to SDCC library
functions.

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

src/board/system76/common/main.c

@@ -102,7 +102,7 @@ void main(void) {
     uint32_t last_time_fan = 0;
 
     for (main_cycle = 0;; main_cycle++) {
-        switch (main_cycle % 3U) {
+        switch (main_cycle & 3U) {
         case 0:
             // Handle USB-C events immediately before power states
             usbpd_event();
@@ -123,6 +123,11 @@ void main(void) {
             // Handle lid close/open
             lid_event();
             break;
+
+        case 3:
+            // We previously used modulo to limit when the logic for other
+            // cases ran, so this case is unused.
+            break;
         }
 
         if (main_cycle == 0) {