arch/stm32: Clear all PWM channel when during STOP

I noticed when executing pwm STOP command in
multichannel mode, the channel still outputting.

This commit fixes this issue.

Signed-off-by: Alan C. Assis <acassis@gmail.com>

arch/arm/src/stm32/stm32_pwm.c

@@ -4463,6 +4463,13 @@ static int pwm_stop(struct pwm_lowerhalf_s *dev)
   outputs = pwm_outputs_from_channels(priv);
   ret = pwm_outputs_enable(dev, outputs, false);
 
+  /* Clear all channels */
+
+  pwm_putreg(priv, STM32_GTIM_CCR1_OFFSET, 0);
+  pwm_putreg(priv, STM32_GTIM_CCR2_OFFSET, 0);
+  pwm_putreg(priv, STM32_GTIM_CCR3_OFFSET, 0);
+  pwm_putreg(priv, STM32_GTIM_CCR4_OFFSET, 0);
+
   leave_critical_section(flags);
 
   pwm_dumpregs(dev, "After stop");