[ALSA] Kill useless volatile in pcm.h

The volatile prefix is just useless there.  Let's kill them, and then
gcc will be happier, too.
   sound/acore/pcm.c:867: warning: passing argument 1 of ‘__constant_c_and_count_memset’ discards qualifiers from pointer target type

Signed-off-by: Takashi Iwai <tiwai@suse.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>