Currently OMAP doesn't define a MAX_DMA_CHANNELS value, which leads to:

  CC      arch/arm/kernel/dma.o
  arch/arm/kernel/dma.c:27:5: warning: "MAX_DMA_CHANNELS" is not defined

It seems like the sensible thing to do is to set this to 0 so this entire
API is just avoided, as everything is using the OMAP-specific one now
anyways.