ALSA: pci: Constify snd_pcm_hardware definitions

Most of snd_pcm_hardware definitions are just copied to another object
as-is, hence we can define them as const for further optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-2-tiwai@suse.de
Signed-off-by: Takashi Iwai <tiwai@suse.de>
diff --git a/sound/pci/echoaudio/indigoiox.c b/sound/pci/echoaudio/indigoiox.c
index f5c9ef6..a017c96 100644
--- a/sound/pci/echoaudio/indigoiox.c
+++ b/sound/pci/echoaudio/indigoiox.c
@@ -62,7 +62,7 @@ static const struct pci_device_id snd_echo_ids[] = {
 	{0,}
 };
 
-static struct snd_pcm_hardware pcm_hardware_skel = {
+static const struct snd_pcm_hardware pcm_hardware_skel = {
 	.info = SNDRV_PCM_INFO_MMAP |
 		SNDRV_PCM_INFO_INTERLEAVED |
 		SNDRV_PCM_INFO_BLOCK_TRANSFER |