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.