PCI: endpoint: Make struct pci_epf_ops in pci_epf_driver const

The pci_epf_ops struct contains a set of callbacks that are used by the
pci_epf_driver, and is never modified by the EPF core itself.

Marking the struct pointer const allows EPF drivers to declare their
pci_epf_ops struct to be const.

This allows the struct to be placed in the read-only section. Which
for example brings some security benefits as the callbacks can not be
overwritten.

[kwilczynski: commit log]
Link: https://lore.kernel.org/linux-pci/20230722230848.589428-1-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>