iio: sx9310: Use long instead of int for channel bitmaps

Uses for_each_set_bit() macro to loop over channel bitmaps.

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