PCI: Add comment about needing pci_msi_off() even when CONFIG_PCI_MSI=n

Per f5f2b131 ("msi: sanely support hardware level msi disabling"), we
want pci_msi_off() to work even if MSI support is not compiled into the
kernel, and there are existing callers that use it when CONFIG_PCI_MSI=n.
This adds a comment to that effect.

No functional change.

Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>