iio: adc: nxp-sar-adc: fix division by zero in write_raw

In the Linux kernel, the following vulnerability has been resolved: iio: adc: nxp-sar-adc: 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 or a negative value to the sampling_frequency sysfs attribute triggers a division by zero in the kernel. Also prevent unsigned integer underflow when the computed cycle count is smaller than NXP_SAR_ADC_CONV_TIME, which would wrap the u32 inpsamp to a huge value.