RDMA/hns: Limit scope of hns_roce_cmq_send()

The forgotten static keyword causes to the following error to appear while
building HNS driver. Declare hns_roce_cmq_send() to be static function to
fix this warning.

drivers/infiniband/hw/hns/hns_roce_hw_v2.c:1089:5: warning: no previous
prototype for _hns_roce_cmq_send_ [-Wmissing-prototypes]
 int hns_roce_cmq_send(struct hns_roce_dev *hr_dev,

Fixes: 6a04aed6 ("RDMA/hns: Fix the chip hanging caused by sending mailbox&CMQ during reset")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>