RDMA/hns: Remove set but not used variable 'rst'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/infiniband/hw/hns/hns_roce_hw_v2.c: In function 'hns_roce_v2_qp_flow_control_init':
drivers/infiniband/hw/hns/hns_roce_hw_v2.c:4384:33: warning:
 variable 'rst' set but not used [-Wunused-but-set-variable]

It never used since introduction.

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>