hw/ac97: Mask out unused bits of volume controls

hw/ac97: Mask out unused bits of volume controls

The Linux ac97 drivers does a number of register read/write tests to
see how much resolution a volume control actually has.

This patch takes this into account by masking out any bits written to
a volume control reg which should not be there according to the spec.

After this the Linux ac97 driver correctly uses a range of 0 - 0x1f for
the PCM out volume, as stated in the spec, and we can fix the FIXME
in update_combined_volume_out().

This patch was also tested with a Windows XP guest without any issues.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: malc <av1474@comtv.ru>