staging: iio: ad7476: fix !CONFIG_IIO_RING_BUFFER build issue reported by Randy Dunlap

Fix function function decalration
Enable CONFIG_IIO_RING_BUFFER for the ad7476 driver
Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>

drivers/staging/iio/adc/Kconfig

@@ -50,6 +50,9 @@ config AD799X_RING_BUFFER
 config AD7476
 	tristate "Analog Devices AD7475/6/7/8 AD7466/7/8 and AD7495 ADC driver"
 	depends on SPI
+	select IIO_RING_BUFFER	
+	select IIO_SW_RING
+	select IIO_TRIGGER 	
 	help
 	  Say yes here to build support for Analog Devices
 	  AD7475, AD7476, AD7477, AD7478, AD7466, AD7467, AD7468, AD7495

drivers/staging/iio/adc/ad7476.h

@@ -59,9 +59,7 @@ int ad7476_scan_from_ring(struct ad7476_state *st);
 int ad7476_register_ring_funcs_and_init(struct iio_dev *indio_dev);
 void ad7476_ring_cleanup(struct iio_dev *indio_dev);
 #else /* CONFIG_IIO_RING_BUFFER */
-static inline ssize_t ad7476_scan_from_ring(struct device *dev,
-					     struct device_attribute *attr,
-					     char *buf)
+static inline int ad7476_scan_from_ring(struct ad7476_state *st)
 {
 	return 0;
 }