ALSA: opl3: Constify snd_opl3_drum_voice definitions

The snd_opl3_drum_voice items are all read-only, hence they can be
declared as const.

There should be no functional changes by this patch.

Link: https://lore.kernel.org/r/20200103081714.9560-45-tiwai@suse.de


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