ASoC: fsl_audmix: cache pdev->dev pointer

There should be no trouble to understand dev = pdev->dev.
This can save some space to have more print info or save
some wrapped lines.

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Suggested-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>