sound: virtuoso: do not overwrite EEPROM on Xonar D2/D2X

On the Asus Xonar D2 and D2X models, the SPI chip select signal for the
fourth DAC shares its pin with the serial clock for the EEPROM that
contains the PCI subdevice ID values.  It appears that when DAC
registers are written and some other unknown conditions occur (probably
noise on the EEPROM's chip select line), the EEPROM gets overwritten
with garbage, which makes it impossible to properly detect the card
later.

Therefore, we better avoid DAC register writes and make sure that the
driver works with the DAC's registers' default values.  Consequently,
the sample format is now I2S instead of left-justified (no user-visible
change), and the DAC's volume/mute registers cannot be used anymore
(volume changes are now done by the software volume plugin).

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