dmaengine: sa11x0: unexport sa11x0_dma_filter_fn and clean up

As we now have no users of sa11x0_dma_filter_fn() in the tree, we can
unexport this function, and remove the now unused header file.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Vinod Koul <vkoul@kernel.org>