staging: android: ion: Remove dead code in ion_page_pool_free

ion_page_pool_add will always return 0, however ion_page_pool_free will
call ion_page_pool_free_pages when ion_page_pool_add's return value is
not 0, so it is a dead code which can be removed.

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