This is useful in situations where the ThreadSafeFlag is reused and needs
to be cleared of any previous, unwanted event.
For example, clear the flag at the start of an operation, trigger the
operation (eg an I2C write), then (a)wait for an external event to set the
flag (eg a pin IRQ). Further events may trigger the flag again but these
are unwanted and should be cleared before the next cycle starts.
