ALSA: bt87x: Constify snd_bt87x_boards

The snd_bt87x_boards array is referred as read-only, hence it can be
declared as const gracefully.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-54-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

sound/pci/bt87x.c

@@ -150,7 +150,7 @@ struct snd_bt87x_board {
 	unsigned no_digital:1;	/* No digital input */
 };
 
-static struct snd_bt87x_board snd_bt87x_boards[] = {
+static const struct snd_bt87x_board snd_bt87x_boards[] = {
 	[SND_BT87X_BOARD_UNKNOWN] = {
 		.dig_rate = 32000, /* just a guess */
 	},