ds2781_battery: Convert to module_platform_driver

module_platform_driver can be used to replace the platform_driver
register and unregister functions, with the calls to module_init
and module_exit,

i.e. all the code that is doing like the below

static int __init mymod_init(void)
{
	return platform_driver_register(&drv_operations);
}

static void __exit mymod_exit(void)
{
	platform_driver_unregister(&drv_operations);
}

module_init(mymod_init);
module_exit(mymod_exit);

can be replaced with
module_platform_driver(drv_operations)...

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