dmaengine: edma: make edma_filter_fn private

With the audio driver no longer referring to this function, it
can be made private to the dmaengine driver itself, and the
header file removed.

Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20190722081705.2084961-2-arnd@arndb.de
Signed-off-by: Vinod Koul <vkoul@kernel.org>