iio: sx9310: Fix irq handling

Fixes enable/disable irq handling at various points. The driver needs to
only enable/disable irqs if there is an actual irq handler installed.

Signed-off-by: Daniel Campello <campello@chromium.org>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>