iio: chemical: scd30: fix division by zero in write_raw

In the Linux kernel, the following vulnerability has been resolved:

iio: chemical: scd30: fix division by zero in write_raw

Add a zero check for val2 before using it as a divisor when setting the

sampling frequency. A user writing a zero fractional part to the

sampling_frequency sysfs attribute triggers a division by zero in the

kernel.