R-Car sound DMA will be used from SSI/SRC.
dma.c doesn't alloc DMA handler in .probe timing, because we don't
know what kind of DMA transfer will be used then.
Thus, SSI/SRC have *rsnd_mod for DMA. rsnd_dma_attach() will allocate
it and attach it to system.
It will be PIO mode if it can't alloc DMA handler.

In case of MIX is used, rsnd_dma_attach() will be called twice from SSI.
To avoid duplicate allocation, current rsnd_dma_attach() is checking
allocated DMA handler. This DMA related operation is a little bit
difficult to understand.
This patch adds new rsnd_dma_alloc() and separates allocation and attach
for readable code.
Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Mark Brown <broonie@kernel.org>