PCI: endpoint: pci-epf-ntb: Make struct pci_epf_ops const

The pci_epf_ops struct for the PCI endpoint NTB driver is never modified.

Mark it as const so it can be placed in the read-only section.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20230722230848.589428-3-lars@metafoo.de
Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Krzysztof Wilczyński <kwilczynski@kernel.org>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>