platform/technic_hub: fix race condition in TIM2 irq handler

This fixes a race condition where an interrupt status flag is set while
the interrupt handler is running. We were clearing all flags each time
the handler ran but now we only clear the flags we have handled.

This was triggered when CCR1 was set to 3, causing the green LED to turn
on and stay on all of the time.

Fixes: pybricks/support#224

Author: dlech

lib/pbio/platform/technic_hub/platform.c

@@ -205,12 +205,14 @@ const pbdrv_pwm_stm32_tim_platform_data_t
 // the GPIOs in the timer interrupt handler.
 void TIM2_IRQHandler(void) {
     uint32_t sr = TIM2->SR;
+    uint32_t handled = TIM_SR_UIF;
 
     // green LED
     if (TIM2->CCR1 == 0 || sr & TIM_SR_CC1IF) {
         // If channel 1 duty cycle is 0 or we have reached the CC1 count,
         // turn the GPIO off
         GPIOA->BRR = GPIO_BRR_BR11;
+        handled |= TIM_SR_CC1IF;
     } else if (sr & TIM_SR_UIF) {
         // otherwise if it is the start of the next PWM period turn the GPIO on
         GPIOA->BSRR = GPIO_BSRR_BS11;
@@ -221,13 +223,14 @@ void TIM2_IRQHandler(void) {
         // If channel 2 duty cycle is 0 or we have reached the CC2 count,
         // turn the GPIO off
         GPIOB->BRR = GPIO_BRR_BR15;
+        handled |= TIM_SR_CC2IF;
     } else if (sr & TIM_SR_UIF) {
         // otherwise if it is the start of the next PWM period turn the GPIO on
         GPIOB->BSRR = GPIO_BSRR_BS15;
     }
 
     // clear interrupts
-    TIM2->SR = 0;
+    TIM2->SR &= ~handled;
 }
 
 // Reset