cpu/stm32/periph/timer: prevnt spurious IRQs

[Enoch247]

Contribution description

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

I did not actually observe spurious IRQs happening, but this PR comes as the result of a deep dive into the timer code to solve the bug in #20924. This PR is a defensive measure only.

Testing procedure

Timers should continue to work.

Issues/PRs references

• #20924
• #20925