ALSA: control: Make snd_ctl_find_id() argument const

The id object passed to snd_ctl_find_id() is only read, and we can
mark it with const gracefully.

Link: https://lore.kernel.org/r/20230718141304.1032-9-tiwai@suse.de


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