iio: stk3310: Don't return error code in interrupt handler

commit 8e1eeca5 upstream.

Interrupt handlers must return one of the irqreturn_t values. Returning a
error code is not supported.

The stk3310 event interrupt handler returns an error code when reading the
flags register fails.

Fix the implementation to always return an irqreturn_t value.

Fixes: 3dd477ac

 ("iio: light: Add threshold interrupt support for STK3310")
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Link: https://lore.kernel.org/r/20211024171251.22896-3-lars@metafoo.de
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>