IIO trigger handlers need to return one of the irqreturn_t values.
Returning an error code is not supported.

The ltr501 interrupt handler gets this right for most error paths, but
there is one case where it returns the error code.

In addition for this particular case the trigger handler does not call
`iio_trigger_notify_done()`. Which when not done keeps the triggered
disabled forever.

Modify the code so that the function returns a valid irqreturn_t value as
well as calling `iio_trigger_notify_done()` on all exit paths.

Fixes: 2690be90 ("iio: Add Lite-On ltr501 ambient light / proximity sensor driver")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211024171251.22896-1-lars@metafoo.de
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>