ALSA: uapi: use C90 comment style instead of C99 style

UAPI headers are built with compiler option for C90, thus double-slashes
comment introduced in C99 is not preferable.

Fixes: fb6723da

 ("ALSA: pcm: comment about relation between msbits hw parameter and [S|U]32 formats")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211213081257.36097-1-o-takashi@sakamocchi.jp


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