ASoC: wm8961 - clear WM8961_DACSLOPE bit for normal mode

DACSLOPE bit of Register 06h ADC and DAC Control 2:
        0: Normal mode
        1: Sloping stop-band mode

Thus in the case of normal mode, we should clear DACSLOPE bit.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: stable@kernel.org