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 */