power-supply: Use PTR_ERR_OR_ZERO() routine

At multiple places we are doing exactly what PTR_ERR_OR_ZERO() does. And so that
routine can be reused instead of increasing lines of code here.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>