ASoC: Fix null dereference in ak4535_remove()

ak4535_remove() from sound/soc/codecs/ak4535.c calls
i2c_unregister_device() with a possibly null pointer.

This bug was found by smatch (http://repo.or.cz/w/smatch.git/).

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>