samples: drivers: adc: adc_sequence: Do not overwrite vref_mv

Sample is reading vref-mv property to get reference voltage and then if
reference source is set to ADC_REF_INTERNAL this value is overwritten.
If vref-mv property is provided then it should not be overwritten.

Signed-off-by: Krzysztof Chruściński <krzysztof.chruscinski@nordicsemi.no>