ALSA: line6: Constify snd_ratden definitions

The snd_ratden definitions used in line6 drivers are all read-only, so
they can be marked as const.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-51-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/usb/line6/podhd.c

@@ -42,7 +42,7 @@ struct usb_line6_podhd {
 
 #define line6_to_podhd(x)	container_of(x, struct usb_line6_podhd, line6)
 
-static struct snd_ratden podhd_ratden = {
+static const struct snd_ratden podhd_ratden = {
 	.num_min = 48000,
 	.num_max = 48000,
 	.num_step = 1,