mlxsw: spectrum: qdiscs: Make function mlxsw_sp_qdisc_prio_unoffload static

Fixes the following sparse warning:

drivers/net/ethernet/mellanox/mlxsw/spectrum_qdisc.c:464:1: warning:
 symbol 'mlxsw_sp_qdisc_prio_unoffload' was not declared. Should it be static?

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