stm32/timer: Fix Timer.freq() calc so mult doesn't overflow uint32_t.

Fixes issue #5280.

ports/stm32/timer.c

@@ -1269,15 +1269,21 @@ STATIC mp_obj_t pyb_timer_freq(size_t n_args, const mp_obj_t *args) {
         uint32_t prescaler = self->tim.Instance->PSC & 0xffff;
         uint32_t period = __HAL_TIM_GET_AUTORELOAD(&self->tim) & TIMER_CNT_MASK(self);
         uint32_t source_freq = timer_get_source_freq(self->tim_id);
-        uint32_t divide = ((prescaler + 1) * (period + 1));
+        uint32_t divide_a = prescaler + 1;
+        uint32_t divide_b = period + 1;
         #if MICROPY_PY_BUILTINS_FLOAT
-        if (source_freq % divide != 0) {
-            return mp_obj_new_float((float)source_freq / (float)divide);
-        } else
-        #endif
-        {
-            return mp_obj_new_int(source_freq / divide);
+        if (source_freq % divide_a != 0) {
+            return mp_obj_new_float((mp_float_t)source_freq / (mp_float_t)divide_a / (mp_float_t)divide_b);
         }
+        source_freq /= divide_a;
+        if (source_freq % divide_b != 0) {
+            return mp_obj_new_float((mp_float_t)source_freq / (mp_float_t)divide_b);
+        } else {
+            return mp_obj_new_int(source_freq / divide_b);
+        }
+        #else
+        return mp_obj_new_int(source_freq / divide_a / divide_b);
+        #endif
     } else {
         // set
         uint32_t period;

tests/pyb/timer.py

@@ -11,3 +11,9 @@ tim.prescaler(300)
 print(tim.prescaler())
 tim.period(400)
 print(tim.period())
+
+# Setting and printing frequency
+tim = Timer(2, freq=100)
+print(tim.freq())
+tim.freq(0.001)
+print(tim.freq())

tests/pyb/timer.py.exp

@@ -2,3 +2,5 @@
 200
 300
 400
+100
+0.001