staging: android: check for error from register_shrinker in ion_heap_init_shrinker

The function register_shrinker in ion_heap_init_shrinker may return an
error, check it out. Meanwhile, ion_heap_init_shrinker has to a return
value.

Signed-off-by: Xiongwei Song <sxwjean@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>