iio: core: Fix an error pointer vs NULL bug in devm_iio_device_alloc()

The devm_iio_device_alloc() function is supposed to return NULL and not
error pointers.  Returning an error pointer will lead to a crash in the
callers.

Fixes: cf5724e9 ("iio: core: simplify some devm functions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Alexandru Ardelean <ardeleanalex@gmail.com>
Link: https://lore.kernel.org/r/YJ+a1yaMu2QNATgt@mwanda


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>