regulator: Use _cansleep() for WM8994 regulator GPIOs

The WM8994 regulator driver is perfectly happy if the GPIO used to enable
the regulator sleeps so call the appropriate GPIO API.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>