ALSA: docs: Constify snd_device_ops definitions

Update the documentation for adding the const prefix to snd_device_ops
definitions as well.

Link: https://lore.kernel.org/r/20200103081714.9560-20-tiwai@suse.deSigned-off-by: Takashi Iwai <tiwai@suse.de>

Documentation/sound/kernel-api/writing-an-alsa-driver.rst

@@ -259,7 +259,7 @@ to details explained in the following section.
       {
               struct mychip *chip;
               int err;
-              static struct snd_device_ops ops = {
+              static const struct snd_device_ops ops = {
                      .dev_free = snd_mychip_dev_free,
               };
 
@@ -675,7 +675,7 @@ low-level device with a specified ``ops``,
 
 ::
 
-  static struct snd_device_ops ops = {
+  static const struct snd_device_ops ops = {
           .dev_free =        snd_mychip_dev_free,
   };
   ....
@@ -761,7 +761,7 @@ destructor and PCI entries. Example code is shown first, below.
       {
               struct mychip *chip;
               int err;
-              static struct snd_device_ops ops = {
+              static const struct snd_device_ops ops = {
                      .dev_free = snd_mychip_dev_free,
               };