ALSA: uapi: Fix a C++ style comment in asound.h

UAPI header should have no C++ style comment but only in the
traditional C style comment, but there is still one place we used it
mistakenly.  This patch corrects it.

Fixes: 54228356

 ("ALSA: ctl: remove unused macro for timestamping of elem_value")
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211018114035.18433-1-tiwai@suse.de


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