ASoC: checking kzalloc() for IS_ERR() instead of NULL

There is a typo here that got copy and pasted to several probe
functions.  kzalloc() returns NULL on allocation failures and not an
ERR_PTR.

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