machine: [rp2] discount scheduling delays in I2C timeouts

[eliasnaur]

The gosched call introduces arbitrary long delays in general, and in TinyGo particular because the goroutine scheduler is cooperative and doesn't preempt busy (e.g. compute-heavy) goroutines. This change discounts such delays from the rp2xxx I2C timeout logic.

I tested this change by simulating a busy goroutine:

go func() {
	for {
		// Busy.
		before := time.Now()
		for time.Since(before) < 100*time.Millisecond {
		}
		// Sleep.
		time.Sleep(100 * time.Millisecond)
	}
}()

and testing that I2C transfers would no longer time out.