dmaengine: omap-dma: make omap_dma_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>
Link: https://lore.kernel.org/lkml/20190307151646.1016966-1-arnd@arndb.de/
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Link: https://lore.kernel.org/r/20190722081705.2084961-1-arnd@arndb.de
Signed-off-by: Vinod Koul <vkoul@kernel.org>