The STM32 periph_timer driver reads the timer's status flags, then
clears them all. It is possible that a timer interrupt could occur
between reading the flag and clearing it. This would lead to a lost
interrupt.
The timer's status flags can be cleared by software, but can only be set
by the hardware. This patch takes advantage of this by only clearing the
flags it knows are set. The rest of the flags are set, which doesn't
actually change their state.
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