STM32 timers: avoid RMW when clearing interrupt flags

All defined bits are rc_w0.

The paranoid version of this would write 0 to the reserved bits (0 is the
"reset value"), but this would require knowing which flags are valid on the
actual platform, and adding the corresponding macros.

lib/stm32/common/timer_common_all.c

@@ -280,7 +280,8 @@ tim_reg_base
 
 void timer_clear_flag(uint32_t timer_peripheral, uint32_t flag)
 {
-	TIM_SR(timer_peripheral) &= ~flag;
+	/* All defined bits are rc_w0 */
+	TIM_SR(timer_peripheral) = ~flag;
 }
 
 /*---------------------------------------------------------------------------*/