stm32/timer: Fix use of timer channel callback() method on L4 MCUs.

Since L4 HAL version 1.17.0, HAL_TIM_IC_Start_IT() checks whether specified
channel of timer is busy or not, which is the case if this function is
called more than once without first calling HAL_TIM_IC_Stop_IT().  The fix
in this commit is to call the stop function before calling start.  The PWM
and OC modes have the same issue with the same fix.

Fixes issue #8732.

ports/stm32/timer.c

@@ -1576,6 +1576,7 @@ STATIC mp_obj_t pyb_timer_channel_callback(mp_obj_t self_in, mp_obj_t callback)
         switch (self->mode) {
             case CHANNEL_MODE_PWM_NORMAL:
             case CHANNEL_MODE_PWM_INVERTED:
+                HAL_TIM_PWM_Stop_IT(&self->timer->tim, TIMER_CHANNEL(self));
                 HAL_TIM_PWM_Start_IT(&self->timer->tim, TIMER_CHANNEL(self));
                 break;
             case CHANNEL_MODE_OC_TIMING:
@@ -1584,9 +1585,11 @@ STATIC mp_obj_t pyb_timer_channel_callback(mp_obj_t self_in, mp_obj_t callback)
             case CHANNEL_MODE_OC_TOGGLE:
             case CHANNEL_MODE_OC_FORCED_ACTIVE:
             case CHANNEL_MODE_OC_FORCED_INACTIVE:
+                HAL_TIM_OC_Stop_IT(&self->timer->tim, TIMER_CHANNEL(self));
                 HAL_TIM_OC_Start_IT(&self->timer->tim, TIMER_CHANNEL(self));
                 break;
             case CHANNEL_MODE_IC:
+                HAL_TIM_IC_Stop_IT(&self->timer->tim, TIMER_CHANNEL(self));
                 HAL_TIM_IC_Start_IT(&self->timer->tim, TIMER_CHANNEL(self));
                 break;
         }