ALSA: ess1688: fix OPL3 port setting

The ess1688 driver uses the same port
for PCM audio (SB compatible) and OPL3
synthesis. It is not always right so allow to
choose a different port for OPL3 synthesis.

Signed-off-by: Krzysztof Helt <krzysztof.h1@wp.pl>
Signed-off-by: Takashi Iwai <tiwai@suse.de>