ASoC: atmel: sam9x5_wm8731: fix oops when unload module

As the priv is not assigned to card->drvdata, it is NULL, so when
unload module, it will cause NULL pointer oops.
Assign priv to card->drvdata to fix this issue.

Signed-off-by: Bo Shen <voice.shen@atmel.com>
Signed-off-by: Mark Brown <broonie@linaro.org>