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

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.

Fixes: 4434072a893e ("iio: adc: Add the NXP SAR ADC support for the s32g2/3 platforms")
Signed-off-by: Antoniu Miclaus <antoniu.miclaus@analog.com>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>