CVE-2026-63935: 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 writeraw

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 samplingfrequency sysfs attribute triggers a division by zero in the kernel.

Also prevent unsigned integer underflow when the computed cycle count is smaller than NXPSARADCCONVTIME, which would wrap the u32 inpsamp to a huge value.