ASoC: Disable pulls on WM8994 AIF2 when starting it

Pull control is availalbe for WM8994 AIF2, generally disabled as part of
the GPIO configuration in order to save power after system startup. As on
newer devices in the series there is no GPIO functionality on these pins
this will happen less naturally so have the driver disable the pulls as the
AIF is probed.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>