Spinning for the correct time has the side effect that it may cause
infinite recursion if the callback function calls timer_set.
timer_set->callback->...->timer_set->callback->...->timer_set-> infinity
In theory, the drawback is that the callback for very short timeouts
(<2 lptmr ticks) may be delayed up to 2 lptmr ticks (61 µs)
In practice however, tests performed using tests/bench_timers shows that
this change only affects the accuracy of the timer target when timer_set
is called with a timeout of 0, which results in a delay of 30 µs.
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