iio: adc: vf610_adc: make use of iio_get_masklength()

Use iio_get_masklength() to access '.masklength' so it can be annotated
as __private when there are no more direct users of it.
Signed-off-by: Nuno Sa <nuno.sa@analog.com>
Link: https://patch.msgid.link/20240726-dev-iio-masklength-private3-v1-19-82913fc0fb87@analog.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

drivers/iio/adc/vf610_adc.c

@@ -752,7 +752,7 @@ static int vf610_adc_buffer_postenable(struct iio_dev *indio_dev)
 	writel(val, info->regs + VF610_REG_ADC_GC);
 
 	channel = find_first_bit(indio_dev->active_scan_mask,
-						indio_dev->masklength);
+				 iio_get_masklength(indio_dev));
 
 	val = VF610_ADC_ADCHC(channel);
 	val |= VF610_ADC_AIEN;