PCI/MSI: Move MSI/MSI-X flags updaters to msi.c

PCI/MSI: Move MSI/MSI-X flags updaters to msi.c

pci_msi_set_enable() and pci_msix_clear_and_set_ctrl() are only used from
msi.c, so move them from drivers/pci/pci.h to msi.c.  No functional change
intended.

Link: https://lore.kernel.org/r/20201203185110.1583077-3-helgaas@kernel.org
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Thierry Reding <treding@nvidia.com>