ALSA: usb: 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/20200103081714.9560-3-tiwai@suse.de


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