ASoC: wm8997: Improve EQ coefficient controls

The EQ coefficient binary controls overlapped with the volume controls
for the B4 and B5 volumes, which were controllable from either the
coefficient control or the volume control itself.

This patch adds controls for the mode and moves the coefficient control
to only cover the coefficients.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Mark Brown <broonie@linaro.org>