bcm2835-dma: Derive slave DMA addresses correctly

Slave addresses for DMA are meant to be supplied as physical addresses
(contrary to what struct snd_dmaengine_dai_dma_data does). It is up to
the DMA controller driver to perform the translation based on its own
view of the world, as described in Device Tree.

Now that the Pi Device Trees have the correct peripheral mappings,
replace the hacky address munging with phys_to_dma().

Signed-off-by: Phil Elwell <phil@raspberrypi.com>