dmaengine: mmp_pdma: Remove mmp_pdma_filter_fn()

It's not used anywhere -- drop it.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210121110356.1768635-2-lkundrak@v3.sk


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