IB/hns: Annotate iomem pointers correctly

This patch avoids that sparse complains that there is an address
space mismatch.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Cc: Lijun Ou <oulijun@huawei.com>
Cc: Wei Hu (Xavier) <xavier.huwei@huawei.com>
Cc: Shaobo Xu <xushaobo2@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>