ALSA: es1968: Constify snd_es1968_tea575x_gpios

The snd_es1968_tea575x_gpios table 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/20200103081714.9560-56-tiwai@suse.de


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