iio: gyro: adis16260: fix division by zero in write_raw

iio: gyro: adis16260: fix division by zero in write_raw

Add a validation check for the sampling frequency value before using it
as a divisor. A user writing zero to the sampling_frequency sysfs
attribute triggers a division by zero in the kernel.

Fixes: 089a41985c6c ("staging: iio: adis16260 digital gyro driver")
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Reviewed-by: Nuno Sá <nuno.sa@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>