ASoC: wm8960: Add DAC filter characteristics selection

Support DAC filter characteristics selection: Normal mode
and Sloping stopband. Sloping stopband may have
better frequency response.

Signed-off-by: Shengjiu Wang <shengjiu.wang@nxp.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Link: https://lore.kernel.org/r/1689927842-21165-1-git-send-email-shengjiu.wang@nxp.com


Signed-off-by: Mark Brown <broonie@kernel.org>