Staging: iio: Fixed a punctuation and a spelling mistake.

Added a missing comma and changed 'it it useful' to 'it is useful'.

Signed-off-by: Ankit Baluni <b18007@students.iitmandi.ac.in>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>