ALSA: Remove VOLATILE flag from chmap ctls

The VOLATILE flag was added to control elements by
snd_pcm_add_chmap_ctls() just because I didn't want to have a
side-effect of "alsactl restore".  But now the set operation doesn't
allow to change the value unless the PCM stream is in PREAPRED state,
there is no reason to keep this flag.  Let's rip it off.

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