ASoC: fsl: fix initialization of DMA buffers

The DMA (PCM) driver used by some Freescale PowerPC supports separate DAIs
for playback and capture, so DMA buffers should be allocated only for the
initialized streams.  Instead of checking for the number of active channels,
which apparently is not reliable, check to see if the actual stream object
exists.

Also provide a better name for the DMA interrupt.

Signed-off-by: Timur Tabi <timur@freescale.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>