iio: hid-sensor-attributes: Check sample_frequency/hysteresis write data legitimacy

Neither sample frequency value nor hysteresis value can be set to be a
negative number, check and return "Invalid argument" if they are negative.

If not do this change, sample_frequency will be set into some unknown
value, read hysteresis value after write negative number will return
"Invalid argument".

Signed-off-by: Song Hongyan <hongyan.song@intel.com>
Acked-by: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>