dmaengine: Add dma_get_slave_caps() inline stub when !CONFIG_DMA_ENGINE

Commit 0d5484b1

 ("dmaengine: Move dma_get_slave_caps()
implementation to dmaengine.c") turned the inline dma_get_slave_caps()
function into an external function without adding an inline stub for the
cases where CONFIG_DMA_ENGINE isn't set. This breaks compilation of
drivers using the DMA engine API when CONFIG_DMA_ENGINE isn't set.

Add an inline stub to fix compilation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Fixes: 0d5484b1

 ("dmaengine: Move dma_get_slave_caps() implementation to dmaengine.c")
Signed-off-by: Vinod Koul <vinod.koul@intel.com>