jz4740-battery: Use devm_kzalloc

This is just a small cleanup, makes a logic a little bit more simplier.

Signed-off-by: Marcos Paulo de Souza <marcos.souza.org@gmail.com>
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>