ab8500_charger: Use devm_regulator_get API

devm_regulator_get() is device managed and makes error handling and code
cleanup simpler.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Acked-by: Arun Murthy <arun.murthy@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Anton Vorontsov <anton@enomsg.org>