This flag is redundant and essentially a no-op, as it is set when the
device supports at least playback or recording, which is almost always
the case. But even if the device is mixer-only (i.e., 0 channels), there
is no reason to keep this flag; it is only used to bail out of the vchan
sysctl handlers, but we already bail out anyway if we try to use the
sysctl in a vchan direction that is not supported.
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