Add linear plugin wrapping iec958 PCM for ice1724-based boards

Add linear plugin wrapping iec958 PCM for ice1724-based boards

The ice1724-based cards can handle only 32bit while the apps almost
expet 16bit format for SPDIF I/O.  This prevents the default config
working on many apps like mplayer, xine, etc.

This patch simply adds the least automatic conversion by linear plugin.
Note that "plug" isn't used here.  Otherwise we get a problem of the
routing (plug over plug is buggy).

Signed-off-by: Takashi Iwai <tiwai@suse.de>