ALSA: firewire: Fix C++ style comments in uapi header

UAPI headers are built with -std=c90 and C++ style comments are
explicitly prohibited.  The recent commit overlooked the rule and
caused the error at header installation.  This patch corrects those.

Fixes: bea36afa ("ALSA: firewire-motu: add message parser to gather meter information in register DSP model")
Fixes: 90b28f3b ("ALSA: firewire-motu: add message parser for meter information in command DSP model")
Fixes: 634ec0b2

 ("ALSA: firewire-motu: notify event for parameter change in register DSP model")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20211018113812.0a16efb0@canb.auug.org.au
Link: https://lore.kernel.org/r/20211018063700.30834-1-tiwai@suse.de


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