iio: convert to common i2c_check_functionality() return value

Previously most drivers that used a i2c_check_functionality() check
condition required various error codes on failure. This patchset
converts to a standard of -EOPNOTSUPP

Signed-off-by: Matt Ranostay <mranostay@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
diff --git a/drivers/iio/potentiometer/mcp4531.c b/drivers/iio/potentiometer/mcp4531.c
index a3f6687..0db67fe 100644
--- a/drivers/iio/potentiometer/mcp4531.c
+++ b/drivers/iio/potentiometer/mcp4531.c
@@ -159,7 +159,7 @@
 	if (!i2c_check_functionality(client->adapter,
 				     I2C_FUNC_SMBUS_WORD_DATA)) {
 		dev_err(dev, "SMBUS Word Data not supported\n");
-		return -EIO;
+		return -EOPNOTSUPP;
 	}
 
 	indio_dev = devm_iio_device_alloc(dev, sizeof(*data));