PCI/MSI: Move MSI/MSI-X init to msi.c

PCI/MSI: Move MSI/MSI-X init to msi.c

Move pci_msi_setup_pci_dev(), which disables MSI and MSI-X interrupts, from
probe.c to msi.c so it's with all the other MSI code and more consistent
with other capability initialization.  This means we must compile msi.c
always, even without CONFIG_PCI_MSI, so wrap the rest of msi.c in an #ifdef
and adjust the Makefile accordingly.  No functional change intended.

Link: https://lore.kernel.org/r/20201203185110.1583077-2-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>