ALSA: opl3: Declare common variables properly

Move the declarations of common variables into opl3_voice.h instead of
declaring at each file multiple times, which was error-prone.

This fixes sparse warnings like:
  sound/drivers/opl3/opl3_synth.c:51:6: warning: symbol 'snd_opl3_regmap' was not declared. Should it be static?
Signed-off-by: Takashi Iwai <tiwai@suse.de>

sound/drivers/opl3/opl3_drums.c

@@ -21,8 +21,6 @@
 
 #include "opl3_voice.h"
 
-extern char snd_opl3_regmap[MAX_OPL2_VOICES][4];
-
 static char snd_opl3_drum_table[47] =
 {
 	OPL3_BASSDRUM_ON,  OPL3_BASSDRUM_ON,  OPL3_HIHAT_ON,	/* 35 - 37 */

sound/drivers/opl3/opl3_lib.c

@@ -31,13 +31,12 @@
 #include <linux/slab.h>
 #include <linux/ioport.h>
 #include <sound/minors.h>
+#include "opl3_voice.h"
 
 MODULE_AUTHOR("Jaroslav Kysela <perex@perex.cz>, Hannu Savolainen 1993-1996, Rob Hooft");
 MODULE_DESCRIPTION("Routines for control of AdLib FM cards (OPL2/OPL3/OPL4 chips)");
 MODULE_LICENSE("GPL");
 
-extern char snd_opl3_regmap[MAX_OPL2_VOICES][4];
-
 static void snd_opl2_command(struct snd_opl3 * opl3, unsigned short cmd, unsigned char val)
 {
 	unsigned long flags;

sound/drivers/opl3/opl3_midi.c

@@ -25,10 +25,6 @@
 #include "opl3_voice.h"
 #include <sound/asoundef.h>
 
-extern char snd_opl3_regmap[MAX_OPL2_VOICES][4];
-
-extern bool use_internal_drums;
-
 static void snd_opl3_note_off_unsafe(void *p, int note, int vel,
 				     struct snd_midi_channel *chan);
 /*

sound/drivers/opl3/opl3_oss.c

@@ -29,8 +29,6 @@ static int snd_opl3_reset_seq_oss(struct snd_seq_oss_arg *arg);
 
 /* operators */
 
-extern struct snd_midi_op opl3_ops;
-
 static struct snd_seq_oss_callback oss_callback = {
 	.owner = 	THIS_MODULE,
 	.open =		snd_opl3_open_seq_oss,

sound/drivers/opl3/opl3_synth.c

@@ -24,6 +24,7 @@
 #include <linux/nospec.h>
 #include <sound/opl3.h>
 #include <sound/asound_fm.h>
+#include "opl3_voice.h"
 
 #if IS_ENABLED(CONFIG_SND_SEQUENCER)
 #define OPL3_SUPPORT_SYNTH

sound/drivers/opl3/opl3_voice.h

@@ -52,4 +52,8 @@ void snd_opl3_free_seq_oss(struct snd_opl3 *opl3);
 #define snd_opl3_free_seq_oss(opl3) /* NOP */
 #endif
 
+extern char snd_opl3_regmap[MAX_OPL2_VOICES][4];
+extern bool use_internal_drums;
+extern struct snd_midi_op opl3_ops;
+
 #endif