ASoC: au1x: use substream stream info directly

PCM_TX/RX are the same as SNDRV_PCM_STREAM_PLAYBACK/CAPTURE.  Use
them directly.

Signed-off-by: Manuel Lauss <manuel.lauss@googlemail.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>