iio: accel: fxls8962af: return IRQ_HANDLED when fifo is flushed

fxls8962af_fifo_flush() will return the samples flushed.
So return IRQ_NONE only if an error is returned.

Fixes: 79e3a5bd

 ("iio: accel: fxls8962af: add hw buffered sampling")
Signed-off-by: Sean Nyekjaer <sean@geanix.com>
Link: https://lore.kernel.org/r/20210817124336.1672169-1-sean@geanix.com
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>