pcm: plug: add SND_PCM_FORMAT_{S, U}20 to linear_preferred_formats

pcm: plug: add SND_PCM_FORMAT_{S, U}20 to linear_preferred_formats

This commit adds the recently added formats SND_PCM_FORMAT_{S,U}20 to
the linear_preferred_formats array in pcm_plug.
Let's give them lower priority than more standard S24 formats but a higher
priority than less typical 3-byte versions.

Signed-off-by: Maciej S. Szmigiero <mail@maciej.szmigiero.name>
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>