staging: iio: tsl2x7x: don't return error in IRQ handler

tsl2x7x_event_handler() could return an error and this could cause the
interrupt to remain masked. We shouldn't return an error in the
interrupt handler so this patch always returns IRQ_HANDLED. An error
will be logged if one occurs.

Signed-off-by: Brian Masney <masneyb@onstation.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>