NTB: ntb_hw_idt: Set NTB_TOPO_SWITCH topology

Since Switchtec patch there has been a new topology added to
the NTB API. It's called NTB_TOPO_SWITCH and dedicated for
PCIe switch chips. Even though topo field isn't used within the
IDT driver much, lets set it for the sake of unification.

Signed-off-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: Jon Mason <jdmason@kudzu.us>