ASoC: au1x: dbdma2: fix oops on soc device removal.

platform_device_unregister() frees resources for us, no need to
do it explicitly.  Fixes an oops when machine code removes the
soc-audio device.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>