dmaengine: Move dma_get_{,any_}slave_channel() to private dmaengine.h

The functions dma_get_slave_channel() and dma_get_any_slave_channel()
are called from DMA engine drivers only.  Hence move their declarations
from the public header file <linux/dmaengine.h> to the private header
file drivers/dma/dmaengine.h.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20200121093311.28639-4-geert+renesas@glider.be


Signed-off-by: Vinod Koul <vkoul@kernel.org>