iio: addac: ad74413r: use ngpio size when iterating over mask

ngpio is the actual number of GPIOs handled by the GPIO chip,
as opposed to the max number of GPIOs.

Fixes: fea251b6 ("iio: addac: add AD74413R driver")
Signed-off-by: Cosmin Tanislav <cosmin.tanislav@analog.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220111074703.3677392-1-cosmin.tanislav@analog.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>