staging:iio-trig-periodic-rtc: Allow to reset frequency to 0

periodic rtc trigger initially has frequency 0

it is possible to change trigger frequency on-the-fly, but it is not
possible to set frequency back to 0

this patch allows to set trigger frequency to 0, thereby disabling
the rtc interrupt

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>