gmtime: avoid a modulus

since WEEK_SECONDS = 7 * DAY_SECONDS, the result is the same

Change-Id: Iec161fc2de626c99c1aabf80ab1d3243eac602d9
diff --git a/firmware/libc/gmtime.c b/firmware/libc/gmtime.c
index b6f75e2..e7ebdf0 100644
--- a/firmware/libc/gmtime.c
+++ b/firmware/libc/gmtime.c
@@ -53,7 +53,7 @@
     int year, i, mday, hour, min;
 
     /* weekday */
-    tm->tm_wday = ((seconds % WEEK_SECONDS) / DAY_SECONDS + 4) % 7;
+    tm->tm_wday = (seconds / DAY_SECONDS + 4) % 7;
 
     /* Year */
     year = 1970;