mlxsw: spectrum: Make offloads stats functions static

The offloads stats functions are local to this file, make them static.

Fixes: fc1bbb0f ('mlxsw: spectrum: Implement offload stats ndo [..]')
Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>