ASoC: imx-audmix: fix object reference leaks in probe

Release the reference to the underlying device taken
by of_find_device_by_node() call.

Signed-off-by: Viorel Suman <viorel.suman@nxp.com>
Reported-by: Julia Lawall <Julia.Lawall@lip6.fr>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>