echomixer: Fix echomixer to work with the new drivers

echomixer: Fix echomixer to work with the new drivers

There is a long standing bug in the drivers for cards with a vmixer because
I overlooked a detail in the c++ generic driver by echoaudio. Those cards
do not have a line-out volume control. It is a virtual control provided by
the generic driver. The bug is harmless because the DSP just ignores the
command to change the volume.
Since that control has been removed, echomixer must be updated. With this
patch it uses the vmixer to fake the line-out volume.

This patch makes echomixer work with the new drivers.

Signed-off-by: Giuliano Pochini <pochini@shiny.it>
Signed-off-by: Takashi Iwai <tiwai@suse.de>