ws2812: Fix number of nops for AVR at 8 MHz (qmk#9559)

* ws2812: Fix number of nops for AVR at 8 MHz

When trying to calculate the number of nops for AVR running at 8 MHz,
the value of `w3` is expected to be negative; however, because `F_CPU`
is defined in tmk_core/avr.mk with the `UL` suffix, the preprocessor
performs its calculations using `unsigned long`, getting a very large
positive number instead of the expected negative number; this then
results in generating code with a huge number of nops.  Fix the broken
calculations by performing a comparison before subtraction, so that the
unsigned number wraparound does not occur.

The keyboard which triggers the problem is `handwired/promethium`; the
buggy code silently compiles, but the resulting timings would be
completely wrong.

* ws2812: Clean up the code after the 8 MHz fix

Remove old code which was unsuccessfully trying to clamp negative w1, w2
and w3 values to 0, and set w1_nops, w2_nops and w3_nops directly.

drivers/avr/ws2812.c

@@ -67,19 +67,27 @@ void ws2812_setleds(LED_TYPE *ledarray, uint16_t number_of_leds) {
 #define w_onecycles (((F_CPU / 1000) * w_onepulse + 500000) / 1000000)
 #define w_totalcycles (((F_CPU / 1000) * w_totalperiod + 500000) / 1000000)
 
-// w1 - nops between rising edge and falling edge - low
-#define w1 (w_zerocycles - w_fixedlow)
-// w2   nops between fe low and fe high
-#define w2 (w_onecycles - w_fixedhigh - w1)
-// w3   nops to complete loop
-#define w3 (w_totalcycles - w_fixedtotal - w1 - w2)
-
-#if w1 > 0
-#    define w1_nops w1
+// w1_nops - nops between rising edge and falling edge - low
+#if w_zerocycles >= w_fixedlow
+#    define w1_nops (w_zerocycles - w_fixedlow)
 #else
 #    define w1_nops 0
 #endif
 
+// w2_nops - nops between fe low and fe high
+#if w_onecycles >= (w_fixedhigh + w1_nops)
+#    define w2_nops (w_onecycles - w_fixedhigh - w1_nops)
+#else
+#    define w2_nops 0
+#endif
+
+// w3_nops - nops to complete loop
+#if w_totalcycles >= (w_fixedtotal + w1_nops + w2_nops)
+#    define w3_nops (w_totalcycles - w_fixedtotal - w1_nops - w2_nops)
+#else
+#    define w3_nops 0
+#endif
+
 // The only critical timing parameter is the minimum pulse length of the "0"
 // Warn or throw error if this timing can not be met with current F_CPU settings.
 #define w_lowtime ((w1_nops + w_fixedlow) * 1000000) / (F_CPU / 1000)
@@ -90,18 +98,6 @@ void ws2812_setleds(LED_TYPE *ledarray, uint16_t number_of_leds) {
 #    warning "Please consider a higher clockspeed, if possible"
 #endif
 
-#if w2 > 0
-#    define w2_nops w2
-#else
-#    define w2_nops 0
-#endif
-
-#if w3 > 0
-#    define w3_nops w3
-#else
-#    define w3_nops 0
-#endif
-
 #define w_nop1 "nop      \n\t"
 #define w_nop2 "rjmp .+0 \n\t"
 #define w_nop4 w_nop2 w_nop2