dmaengine: ti: omap-dma: don't allow a null od->plat pointer to be dereferenced

Currently when the call to dev_get_platdata returns null the driver issues
a warning and then later dereferences the null pointer.  Avoid this issue
by returning -ENODEV error rather when the platform data is null and
change the warning to an appropriate error message.

Addresses-Coverity: ("Dereference after null check")
Fixes: 211010ae ("dmaengine: ti: omap-dma: Pass sdma auxdata to driver and use it")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Vinod Koul <vkoul@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>