ASoC: check for substream not channels_min in pcm engines

This is a follow up on 53dea36c which fixes the other affected
pcm engines.

Description from 53dea36c

:
 Don't rely on the codec's channels_min information to decide wheter or
 not allocate a substream's DMA buffer. Rather check if the substream
 itself was allocated previously.

Without this patch I was seeing null-pointer dereferenc in atmel-pcm.

Signed-off-by: Joachim Eastwood <joachim.eastwood@jotron.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>