ALSA: info: Make snd_info_entry_ops as const

The reference to snd_info_entry_ops is rather read-only, so declare it
as a const pointer.  This allows a bit more optimization.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-29-tiwai@suse.de


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