ds2781_battery: Replace call to kzalloc with devm_kzalloc

With devm_kzalloc there is no need of freeing at the errorpath and also
at unregister it.

Signed-off-by: Devendra Naga <develkernel412222@gmail.com>
Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>