regulator: Fix possible nullpointer dereference in regulator_enable()

In the case where _regulator_enable returns an error it was not checked
if a supplying regulator exists before trying to disable it, leading
to a null pointer-dereference if no supplying regulator existed.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>