ASoC: imx-audmix: register the card on a proper dev

This platform device is registered from "fsl_audmix", which is
its parent device. If use pdev->dev.parent for the priv->card.dev,
the value set by dev_set_drvdata in parent device will be covered
by the value in child device.

Fixes: b86ef536 ("ASoC: fsl: Add Audio Mixer machine driver")
Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Reviewed-by: Daniel Baluta <daniel.baluta@nxp.com>
Link: https://lore.kernel.org/r/1566921315-23402-1-git-send-email-shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>