tests: drivers: adc: adc_dma: increase sampling interval for MCUX ADC16

Increase hardware sampling interval to 30ms for MCUX ADC16. The ADC
callback in the repeated_samplings test takes roughly 27ms to execute,
so a sampling frequency of this interval still allows the ADC test to
verify the ADC is being sampled at a given interval, while avoiding the
pitfalls that result from a kernel timer shorter than the duration the
ADC callback takes to run.

Fixes #58467

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>