ALSA: oxygen: Xonar DG(X): modify capture volume functions

Modify the input_vol_* functions to use the new SPI routines,
There is a new applying function that will be called when
the capture source changed.

Signed-off-by: Roman Volkov <v1ron@mail.ru>
Signed-off-by: Clemens Ladisch <clemens@ladisch.de>