IIO: ti_adc: Avoid double threshold event

The threshold event handler is being called before the FIFO has
actually reached the threshold.

The current code receives a FIFO threshold event, masks the interrupt,
clears the event, and schedules a workqueue. The workqueue is run, it
empties the FIFO, and unmasks the interrupt.

In the above sequence, after the event is cleared, it immediately
retriggers since the FIFO remains beyond the threshold. When the IRQ is
unmasked, this triggered event generates another IRQ. However, as the
FIFO has just been emptied, it is likely to not contain enough samples.

The waits to clear the event until the FIFO has actually been emptied,
in the workqueue. The unmasking and masking of the interrupt remains
unchanged.

Signed-off-by: Russ Dill <Russ.Dill@ti.com>