[nrf fromlist] 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.

Upstream PR #: 87405

Signed-off-by: Krzysztof Chruściński <[email protected]>

Author: nordic-krch

samples/drivers/adc/adc_sequence/src/main.c

@@ -60,7 +60,7 @@ int main(void)
 			printf("Could not setup channel #%d (%d)\n", i, err);
 			return 0;
 		}
-		if (channel_cfgs[i].reference == ADC_REF_INTERNAL) {
+		if ((vrefs_mv[i] == 0) && (channel_cfgs[i].reference == ADC_REF_INTERNAL)) {
 			vrefs_mv[i] = adc_ref_internal(adc);
 		}
 	}