IB/hns: Fix the bug when platform_get_resource() exec fail

This patch mainly fixes the bug with platform_get_resource().
It should return NULL when platform_get_resource() exec fail.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Reviewed-by: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Signed-off-by: Salil Mehta <salil.mehta@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>