ASoC: Tegra: Fix error handling in DMA channel alloc

tegra_dma_allocate_channel() returns NULL on errors, not an error pointer.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>