IB/hns: Validate mtu when modified qp

The mtu should be validated when modify qp,so we check it.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Peter Chen <luck.chen@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>