mfd: wm8994: Silence warning about supplies during deferred probe

Don't confuse user with meaningless warning about the failure in getting
supplies in case of deferred probe.

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>