ALSA: opl4: More constification

Apply const prefix to the static volume table.

Just for minor optimization and no functional changes.

Link: https://lore.kernel.org/r/20200105144823.29547-58-tiwai@suse.deSigned-off-by: NTakashi Iwai <tiwai@suse.de>

sound/drivers/opl4/opl4_synth.c

@@ -248,7 +248,7 @@ static const s16 snd_opl4_pitch_map[0x600] = {
  * Attenuation according to GM recommendations, in -0.375 dB units.
  * table[v] = 40 * log(v / 127) / -0.375
  */
-static unsigned char snd_opl4_volume_table[128] = {
+static const unsigned char snd_opl4_volume_table[128] = {
 	255,224,192,173,160,150,141,134,
 	128,122,117,113,109,105,102, 99,
 	 96, 93, 90, 88, 85, 83, 81, 79,