RDMA/hns: Only assign mtu if IB_QP_PATH_MTU bit is set

Only when the IB_QP_PATH_MTU flag of attr_mask is set
it is valid to assign the mtu field of qp context when
qp type is not GSI and UD.

Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>