ALSA: dummy: make local data static

Add missing prefix to make some local data static.

Spotted by sparse:
  sound/drivers/dummy.c:159:20: warning: symbol 'model_emu10k1' was not declared. Should it be static?
  sound/drivers/dummy.c:165:20: warning: symbol 'model_rme9652' was not declared. Should it be static?
  ....

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