ALSA: opl3sa2: add ZV port control

Add ZV port control switch.

This patch is done after solution
given in the ALSA bug #2872 report.
The patch resolves the issue.

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