iio: backend: fix wrong pointer passed to IS_ERR()

[ Upstream commit fa407631 ]

It should be fwnode_back passed to IS_ERR().

Fixes: c464cc61 ("iio: add child nodes support in iio backend framework")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Link: https://patch.msgid.link/20241028135215.1549-1-yangyingliang@huaweicloud.com


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>