power: supply: charger-manager: simplify return statements

Some trivial improvements on the returning value of the
functions:

 - remove unnecessary goto labels that just return, return
   immediately, instead.

 - do not initialize when not needed.

 - return the value from the calling function that fails instead
   of politically choosing -EINVAL.

Signed-off-by: Andi Shyti <andi@etezian.org>
Signed-off-by: Sebastian Reichel <sre@kernel.org>