ztimer: periph_rtt: ensure RTT_MAX_VALUE is honored

Previously, ztimer would happily set an absolute RTT alarm value that exceeds
RTT's maximum value (though not a longer interval), as the `val` was
simply added to `rtt_get_counter()`.
This commit ensures that the target value wraps around RTT_MAX_VALUE.

Fixes #13920.

sys/ztimer/periph_rtt.c

@@ -19,6 +19,7 @@
  *
  * @}
  */
+#include "assert.h"
 #include "periph/rtt.h"
 #include "ztimer/periph_rtt.h"
 
@@ -50,7 +51,16 @@ static void _ztimer_periph_rtt_set(ztimer_clock_t *clock, uint32_t val)
     }
 
     unsigned state = irq_disable();
-    rtt_set_alarm(rtt_get_counter() + val, _ztimer_periph_rtt_callback, clock);
+
+    /* ensure RTT_MAX_VALUE is (2^n - 1) */
+    static_assert((RTT_MAX_VALUE == UINT32_MAX) ||
+                  !((RTT_MAX_VALUE + 1) & (RTT_MAX_VALUE)),
+                  "RTT_MAX_VALUE needs to be (2^n) - 1");
+
+    rtt_set_alarm(
+        (rtt_get_counter() + val) & RTT_MAX_VALUE, _ztimer_periph_rtt_callback,
+        clock);
+
     irq_restore(state);
 }