ASoC: Intel: sst: only select sst-firmware when DW DMAC is built-in

The previous commit ef3e199a

 ("ASoC: Intel: sst: only use
sst-firmware when DW DMAC is available") does not fix the 0day
building errors thoroughly:

   sound/built-in.o: In function 'dw_dma_remove'
   sound/built-in.o: In function 'dw_dma_probe'

Here we fallback to select sst-firmware only when DW DMAC
is built-in selected. We may need to refactor sst common
driver and split DW related codes to platform driver, but
ATM, this fallback may be the smallest fix.

Please be noticed that after applying this patch, we may
need select DW DMAC manually in DMA driver menu, before
we can prompt and select HSW/BDW and old BYT machines.

Signed-off-by: Jie Yang <yang.jie@intel.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Mark Brown <broonie@kernel.org>