iio: core: Return error for failed read_reg

BugLink: http://bugs.launchpad.net/bugs/1724783

commit 3d62c78a upstream.

If an IIO device returns an error code for a read access via debugfs, it
is currently ignored by the IIO core (other than emitting an error
message). Instead, return this error code to user space, so upper layers
can detect it correctly.
Signed-off-by: Matt Fornero <matt.fornero@mathworks.com>
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Stefan Bader <stefan.bader@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>

drivers/iio/industrialio-core.c

@@ -221,8 +221,10 @@ static ssize_t iio_debugfs_read_reg(struct file *file, char __user *userbuf,
 	ret = indio_dev->info->debugfs_reg_access(indio_dev,
 						  indio_dev->cached_reg_addr,
 						  0, &val);
-	if (ret)
+	if (ret) {
 		dev_err(indio_dev->dev.parent, "%s: read failed\n", __func__);
+		return ret;
+	}
 
 	len = snprintf(buf, sizeof(buf), "0x%X\n", val);