staging: iio: trigger: iio-trig-periodic-rtc: Change frequency type to unsigned int

Change struct iio_prtc_trigger_info frequency
type from int to unsigned int.
Since it is always treated as such in the driver
so they type should probably reflect this.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>