dmaengine: dma_sync_wait and dma_find_channel undefined

dma_sync_wait and dma_find_channel are declared regardless of whether
CONFIG_DMA_ENGINE is enabled, but calling the function without
CONFIG_DMA_ENGINE enabled results "undefined reference" errors.

To get around this, declare dma_sync_wait and dma_find_channel as inline
functions if CONFIG_DMA_ENGINE is undefined.

Signed-off-by: Jon Mason <jon.mason@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>