ASoC: sam9g20_wm8731: fix resource leak in at91sam9g20ek_init error path

Fix the error path to properly free allocated resources.

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