iio:inkern: fix overwritten -EPROBE_DEFER in of_iio_channel_get_by_name

commit 872687f626e033b4ddfaec1e410057cfc6636d77 upstream.

Fixes: a2c12493ed7e ('iio: of_iio_channel_get_by_name() returns non-null pointers for error legs')

which improperly assumes that of_iio_channel_get_by_name must always
return NULL and thus now hides -EPROBE_DEFER.

Signed-off-by: Johannes Pointner <johannes.pointner@br-automation.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>