ALSA: ice1724: Use linear scale for AK4396 volume control.

The AK4396 DAC has a linear-scale attentuator, but
sound/pci/ice1712/prodigy_hifi.c used a log scale instead, which is
not quite right.  This patch restores the correct scale, borrowing
from the ak4396 code in sound/pci/oxygen/oxygen.c.

Signed-off-by: Matteo Frigo <athena@fftw.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>