ASoC: davinci-mcasp: check memory allocation failure

Check memory allocation failures and return -ENOMEM in such cases, as
already done above for another memory allocation.

This avoids NULL pointers dereference.

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Mark Brown <broonie@kernel.org>