ALSA: drivers: mpu401: constify pnp_device_id

pnp_device_id are not supposed to change at runtime. All functions
working with pnp_device_id provided by <linux/pnp.h> work with
const pnp_device_id. So mark the non-const structs as const.
Signed-off-by: NArvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: NTakashi Iwai <tiwai@suse.de>

sound/drivers/mpu401/mpu401.c

@@ -148,7 +148,7 @@ static struct platform_driver snd_mpu401_driver = {
 
 #define IO_EXTENT 2
 
-static struct pnp_device_id snd_mpu401_pnpids[] = {
+static const struct pnp_device_id snd_mpu401_pnpids[] = {
 	{ .id = "PNPb006" },
 	{ .id = "" }
 };