ALSA: sscape: drop redundant fields from soundscape struct

The wss_base is disuised parameter for one function.
It is converted to function parameter.

The code_type is only set but never read.
It is removed.

The midi_vol is set only to 0 so it does not work
as detection of change in midi volume. It is fixed.

The xport variable is alias to the port[dev]. Use
the port[dev] directly to increase readability.

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