mlxsw: spectrum_router: Add proper function documentation

Suppress following warnings when compiling with W=1:

drivers/net/ethernet/mellanox/mlxsw//spectrum_router.c:1552: warning:
Function parameter or member 'mlxsw_sp' not described in
'__mlxsw_sp_ipip_entry_update_tunnel'
drivers/net/ethernet/mellanox/mlxsw//spectrum_router.c:1552: warning:
Function parameter or member 'ipip_entry' not described in
'__mlxsw_sp_ipip_entry_update_tunnel'
drivers/net/ethernet/mellanox/mlxsw//spectrum_router.c:1552: warning:
Function parameter or member 'extack' not described in
'__mlxsw_sp_ipip_entry_update_tunnel'

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>