ALSA: ca0106: Constify snd_ca0106_category_str items

snd_ca0106_con_category array is read-only and can be marked as const.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-47-tiwai@suse.de


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