power: supply: ab8500: Use local helper

Use a local "dev" helper variable to make the probe()
code easier to read in the ab8500 subdrivers.

Drop out-of-memory messages as these should come from the
slab core.

Cc: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>