ASoC: stm32: fix a typo in stm32_adfsdm_probe()

Fix a typo, we should return PTR_ERR(priv->iio_cb) instead of
PTR_ERR(priv->iio_ch).

Fixes: 55da0948 ("ASoC: stm32: add DFSDM DAI support")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Mark Brown <broonie@kernel.org>