mlxsw: spectrum: Add missing DCB rollback in error path

We correctly execute mlxsw_sp_port_dcb_fini() when port is removed, but
I missed its rollback in the error path of port creation, so add it.

Fixes: f00817df

 ("mlxsw: spectrum: Introduce support for Data Center Bridging (DCB)")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reviewed-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>