iio: imu: adis16480: make sure provided frequency is positive

It could happen that either `val` or `val2` [provided from userspace] is
negative. In that case the computed frequency could get a weird value.

Fix this by checking that neither of the 2 variables is negative, and check
that the computed result is not-zero.

Fixes: e4f95939

 ("iio: imu: adis16480 switch sampling frequency attr to core support")
Signed-off-by: Alexandru Ardelean <alexandru.ardelean@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>