Stop PWM before assigning the new sequence

I had introduced a small bug in my last PR where I assigned the sequence before stopping the PWM. I now stop the PWM before doing that now.

Also, corrected a math comment.

embassy-nrf/src/pwm.rs

@@ -149,6 +149,8 @@ impl<'d, T: Instance> SequencePwm<'d, T> {
             return Err(Error::SequenceTimesAtLeastOne);
         }
 
+        self.stop();
+
         let r = T::regs();
 
         r.seq0
@@ -165,8 +167,6 @@ impl<'d, T: Instance> SequencePwm<'d, T> {
             .cnt
             .write(|w| unsafe { w.bits(sequence.len() as u32) });
 
-        self.stop();
-
         r.enable.write(|w| w.enable().enabled());
 
         // defensive before seqstart