DMA: PL330: Check the pointer returned by kzalloc

kzalloc could return NULL. Hence add a check to avoid
NULL pointer dereference.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Stable <stable@vger.kernel.org>
Signed-off-by: Vinod Koul <vinod.koul@linux.intel.com>