PCI: histb: Constify dw_pcie_host_ops structure

The dw_pcie_host_ops structure is only stored in the ops field
of a pcie_port structure, and this field is const, so make the
dw_pcie_host_ops structure const as well.

Done with the help of Coccinelle.

Signed-off-by: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>