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>