cpu/stm32/periph/timer: prevnt spurious IRQs

This patch hardens the STM32 timer driver against some possible causes
of spurious IRQs.

cpu/stm32/periph/timer.c

@@ -195,11 +195,13 @@ int timer_set(tim_t tim, int channel, unsigned int timeout)
     }
 #endif
 
-    /* clear spurious IRQs */
-    dev(tim)->SR &= ~(TIM_SR_CC1IF << channel);
-
     TIM_CHAN(tim, channel) = value;
 
+    /* clear spurious IRQs
+     * note: This might also clear the IRQ just set, but that is handled below
+     * anyway. */
+    dev(tim)->SR &= ~(TIM_SR_CC1IF << channel);
+
     /* enable IRQ */
     dev(tim)->DIER |= (TIM_DIER_CC1IE << channel);
 
@@ -264,7 +266,13 @@ int timer_clear(tim_t tim, int channel)
     }
 
     unsigned irqstate = irq_disable();
+
+    /* disable IRQ */
     dev(tim)->DIER &= ~(TIM_DIER_CC1IE << channel);
+
+    /* clear spurious IRQs */
+    dev(tim)->SR &= ~(TIM_SR_CC1IF << channel);
+
     irq_restore(irqstate);
 
 #ifdef MODULE_PERIPH_TIMER_PERIODIC