iio:inkern: fix overwritten -EPROBE_DEFER in of_iio_channel_get_by_name

Fixes: a2c12493 ('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>
Cc: Stable@vger.kernel.org