[ALSA] virtuoso: correctly switch input jack on Xonar DX

When selecting the capture source on the Xonar DX, the input jack must
be routed to either the line input or the microphone input by setting a
GPIO pin.  This requires an additional callback so that the model driver
can hook into the toggling of AC97 switches.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Takashi Iwai <tiwai@suse.de>