ALSA: oxfw: use const qualifier for immutable argument

In the helper function, the first argument is immutable, thus it is
preferable to use const qualifier.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Link: https://lore.kernel.org/r/20240218074128.95210-2-o-takashi@sakamocchi.jp
Signed-off-by: Takashi Iwai <tiwai@suse.de>