ALSA: spi: Constify snd_kcontrol_new items

Most of snd_kcontrol_new definitions are read-only and passed as-is.
Let's declare them as const for further optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-41-tiwai@suse.de


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