net/mlx5: Make function mlx5_fpga_tls_send_teardown_cmd() static

Fixes the following sparse warning:

drivers/net/ethernet/mellanox/mlx5/core/fpga/tls.c:199:6: warning:
 symbol 'mlx5_fpga_tls_send_teardown_cmd' was not declared. Should it be static?

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>