staging: android: ion: Return void instead of int

Now, nobody care about the return value of ion_page_pool_add, therefore
we can just make it return void.

Acked-by: Laura Abbott <labbott@redhat.com>
Signed-off-by: Yisheng Xie <xieyisheng1@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>