Fix overflows in WPM calculations (qmk#13128)

* Fix overflow in WPM calculations.

First, the "fresh" WPM calculation could end up being up to 12000 (with
default `WPM_ESTIMATED_WORD_SIZE`) if keys were pressed more or less
simultaneously. This value has now been clamped down to 255, in effect
clamping WPM to its max value of 255.

Second, with `WPM_ALLOW_COUNT_REGRESSION` enabled, it was possible to
regress the WPM below 0 (i.e. to 255) by just repeatedly pressing
backspace.

* Fix WPM being limited to 235 due to float/int logic.

quantum/wpm.c

@@ -17,6 +17,8 @@
 
 #include "wpm.h"
 
+#include <math.h>
+
 // WPM Stuff
 static uint8_t  current_wpm = 0;
 static uint16_t wpm_timer   = 0;
@@ -69,14 +71,22 @@ __attribute__((weak)) uint8_t wpm_regress_count(uint16_t keycode) {
 void update_wpm(uint16_t keycode) {
     if (wpm_keycode(keycode)) {
         if (wpm_timer > 0) {
-            current_wpm += ((60000 / timer_elapsed(wpm_timer) / WPM_ESTIMATED_WORD_SIZE) - current_wpm) * wpm_smoothing;
+            uint16_t latest_wpm = 60000 / timer_elapsed(wpm_timer) / WPM_ESTIMATED_WORD_SIZE;
+            if (latest_wpm > UINT8_MAX) {
+                latest_wpm = UINT8_MAX;
+            }
+            current_wpm += ceilf((latest_wpm - current_wpm) * wpm_smoothing);
         }
         wpm_timer = timer_read();
     }
 #ifdef WPM_ALLOW_COUNT_REGRESSION
     uint8_t regress = wpm_regress_count(keycode);
     if (regress) {
-        current_wpm -= regress;
+        if (current_wpm < regress) {
+            current_wpm = 0;
+        } else {
+            current_wpm -= regress;
+        }
         wpm_timer = timer_read();
     }
 #endif