ASoC: omap-hdmi-audio: Add missing error check for devm_kzalloc

This patch add a missing check on the return value of devm_kzalloc,
which would cause a NULL pointer dereference in a OOM situation.

Signed-off-by: Kiran Padwal <kiran.padwal@smartplayin.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>