PCI: endpoint: pci-epf-mhi: Make structs pci_epf_ops and pci_epf_event_ops const

Both the pci_epf_ops and pci_epf_evnt_ops structs for the PCI endpoint
MHI driver are never modified.

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

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20230722230848.589428-2-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>