staging/iio: use module_platform_driver macro

the code which under _init and _exit does only the platform_driver_register
and platform_driver_unregister, and nothing else,
so its better to use the module_platform_driver macro rather duplicating
its implementation

Signed-off-by: Devendra Naga <develkernel412222@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>