commit da019f59 upstream.

When not using the "_optional" function, a dummy regulator is returned
and the driver fails to initialize.
Signed-off-by: Urs Fässler <urs.fassler@bytesatwork.ch>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>