dmaengine: pxa/mmp: stop referencing config->slave_id

The last driver referencing the slave_id on Marvell PXA and MMP platforms
was the SPI driver, but this stopped doing so a long time ago, so the
TODO from the earlier patch can no be removed.

Fixes: b729bf34 ("spi/pxa2xx: Don't use slave_id of dma_slave_config")
Fixes: 13b3006b

 ("dma: mmp_pdma: add filter function")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20211122222203.4103644-7-arnd@kernel.org
Signed-off-by: Vinod Koul <vkoul@kernel.org>