power: suppy: ucs1002: disable power when max current is 0

For some devices userspace needs the ability to completely cut the power
to the USB devices connected to the charge controller. An easy way to
achieve this is by allowing 0 as a valid max current and forcibly disable
the output in that case, as well as enable it again if the regulator is
in use and a non-0 max current is set.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Tested-by: Chris Healy <cphealy@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>