mfd: wm8994: Fix driver operation if loaded as modules

mfd: wm8994: Fix driver operation if loaded as modules

WM8994 chip has built-in regulators, which might be used for chip
operation. They are controlled by a separate wm8994-regulator driver,
which should be loaded before this driver calls regulator_get(), because
that driver also provides consumer-supply mapping for the them. If that
driver is not yet loaded, regulator core substitute them with dummy
regulator, what breaks chip operation, because the built-in regulators are
never enabled. Fix this by annotating this driver with MODULE_SOFTDEP()
"pre" dependency to "wm8994_regulator" module.

Signed-off-by: Marek Szyprowski <m.szyprowski@samsung.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>