spi: tegra: checking for ERR_PTR instead of NULL

dma_request_slave_channel() returns NULL on error and not ERR_PTRs.
I've fixed this by using dma_request_slave_channel_reason() which does
return ERR_PTRs.

Fixes: a915d150 ('spi: tegra: convert to standard DMA DT bindings')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>