PCI: endpoint: Align pci_epc_get_msix(), pci_epc_ops::get_msix() return value encoding

PCI: endpoint: Align pci_epc_get_msix(), pci_epc_ops::get_msix() return value encoding

The kdoc for pci_epc_get_msix() says:
"Invoke to get the number of MSI-X interrupts allocated by the RC"

The kdoc for the callback pci_epc_ops->get_msix() says:
"ops to get the number of MSI-X interrupts allocated by the RC from the
MSI-X capability register"

pci_epc_ops::get_msix() does however return the number of interrupts in the
encoding as defined by the Table Size field. Nowhere in the kdoc does it
say that the returned number of interrupts is in Table Size encoding.

It is very confusing that the API pci_epc_get_msix() and the callback
function pci_epc_ops::get_msix() don't return the same value.

Clean up the API and the callback function to have the same semantics,
i.e. return the number of interrupts, regardless of the internal encoding
of that value.

[bhelgaas: more specific subject]

Signed-off-by: Niklas Cassel <cassel@kernel.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Cc: stable+noautosel@kernel.org # this is simply a cleanup
Link: https://patch.msgid.link/20250514074313.283156-12-cassel@kernel.org