Added a new mixer switch to enable/disable the sharing of the default
PCM stream with analog and SPDIF outputs.  When "IEC958 Default PCM"
switch is on, the PCM stream is routed both to analog and SPDIF outputs.
This is the behavior in the earlier version.

Turning this switch off has a merit for some codecs, though.  Some codec
chips don't support 24bit formats for SPDIF but only for analog outputs.
In this case, you can use 24bit format by disabling this switch.
Signed-off-by: NTakashi Iwai <tiwai@suse.de>