ASoC: arizona: Fix noise generator gain TLV

The Arizona codec drivers had an incorrect dB scaling for the
noise generator gain that started at 0dB and went upwards.
Actually the highest setting is 0dB.

Signed-off-by: Richard Fitzgerald <rf@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org