ASoC: nuc900-ac97: fix a memory leak

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Acked-by: Wan ZongShun <mcuos.com@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>