PCI: dwc: Constify dw_pcie_host_ops structures

The dw_pcie_host_ops structures are never modified.  Constify these
structures such that these can be write-protected.

Signed-off-by: Jisheng Zhang <jszhang@marvell.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Jingoo Han <jingoohan1@gmail.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>