iio: trigger: Replace explicit casting and wrong specifier with proper one

iio: trigger: Replace explicit casting and wrong specifier with proper one

By unknown reason device name is set with an index casted from int
to unsigned long while at the same time with "%ld" specifier. Both parts
seems wrong to me, thus replace replace explicit casting and wrong specifier
with proper one, i.e. "%d".

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210402174911.44408-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>