[PATCH] ASoC: core: use less strict tests for dailink capabilities

[Pierre-Louis Bossart]

> Again, this is changing the original meaning of the flag from "playback
> allowed" to "playback required".
> 
> This patch (or the orignal) does not explain why this change of meaning
> is necessary ? The point I was making here [0] still stands.
> 
> If your evil plan is to get rid of 2 of the 4 flags, why go through the
> trouble of the changing the meaning and effect of one them ?

My intent was to have a non-ambiguous definition.

I don't know 'playback allowed' means. What is the point of using this 
flag if it may or may not accurately describe what is actually 
implemented? And how can we converge the use of flags since in the 
contrary 'playback_only' is actually a clear indication of what the link 
does. We've got to align on the semantics, and I really don't see the 
point of watering-down definitions. When things are optional or poorly 
defined, the confusion continues.

WFIW, my 'evil' plan was to rename 'dpcm_playback' as 'can_playback' 
(same for capture) and replace 'playback_only' by 'can_playback = 1; 
can_capture = 0'. So this first step was really to align them on the 
expected behavior and minimal requirements.