drivers: adc: adc_mcux_adc16: Remove reference check

Only ADC_REF_INTERNAL reference option is allowed in
adc_mcux_adc16 driver, but the reference is not used
for driver configuration. Internal, external or VDD
can be used in reality depending on SoC type and
its configuration.
Remove the reference option check to allow any reference
option from Devicetree.

Signed-off-by: Michal Smola <michal.smola@nxp.com>