IB/hns: Fix return value check in hns_roce_get_cfg()

In case of error, the function devm_ioremap_resource() returns ERR_PTR()
and never returns NULL. The NULL test in the return value check should
be replaced with IS_ERR().

Signed-off-by: Wei Yongjun <weiyj.lk@gmail.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>