Replace the channels_min check with a check for the relevant substream
being present. Suggested here [1] when mxs implemented the
audio-support.

[1] http://www.spinics.net/lists/arm-kernel/msg133010.htmlSigned-off-by: NWolfram Sang <w.sang@pengutronix.de>
Acked-by: NSascha Hauer <s.hauer@pengutronix.de>
Acked-by: NLiam Girdwood <lrg@ti.com>
Signed-off-by: NMark Brown <broonie@opensource.wolfsonmicro.com>