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.de


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