fix bug in rp2040 SetPeriod implementation

Author: soypat

src/machine/machine_rp2040_pwm.go

@@ -260,7 +260,9 @@ func (pwm *pwmGroup) setPeriod(period uint64) error {
 		maxTop = math.MaxUint16
 		// start algorithm at 95% Top. This allows us to undershoot period with prescale.
 		topStart     = 95 * maxTop / 100
-		milliseconds = 1_000_000_000
+		nanosecond   = 1                  // 1e-9 [s]
+		microsecond  = 1000 * nanosecond  // 1e-6 [s]
+		milliseconds = 1000 * microsecond // 1e-3 [s]
 		// Maximum Period is 268369920ns on rp2040, given by (16*255+15)*8*(1+0xffff)*(1+1)/16
 		// With no phase shift max period is half of this value.
 		maxPeriod = 268 * milliseconds