ASoC: codecs: pcm186x: fix wrong usage of DECLARE_TLV_DB_SCALE()

According to DS, the gain is between -12 dB and 40 dB, with a 0.5 dB step.
Tested on pcm1863.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Acked-by: Andrew F. Davis <afd@ti.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Cc: stable@vger.kernel.org