iio: sx9310: Off by one in sx9310_read_thresh()

This > should be >= to prevent reading one element beyond the end of
the sx9310_pthresh_codes[] array.

Fixes: ad2b473e

 ("iio: sx9310: Support setting proximity thresholds")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Link: https://lore.kernel.org/r/X8XqwK0z//8sSWJR@mwanda
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>