staging:iio: Move the ad7298 driver out of staging

The driver does not expose any custom API to userspace and none of the standard
static code checker tools report any issues, so move it out of staging.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/adc/Makefile b/drivers/iio/adc/Makefile
index 54ac7bb..3256dc6 100644
--- a/drivers/iio/adc/Makefile
+++ b/drivers/iio/adc/Makefile
@@ -4,6 +4,7 @@
 
 obj-$(CONFIG_AD_SIGMA_DELTA) += ad_sigma_delta.o
 obj-$(CONFIG_AD7266) += ad7266.o
+obj-$(CONFIG_AD7298) += ad7298.o
 obj-$(CONFIG_AD7476) += ad7476.o
 obj-$(CONFIG_AD7791) += ad7791.o
 obj-$(CONFIG_AD7887) += ad7887.o