The _OSC capability OSC_MSI_SUPPORT is set when the root bridge is added
with pci_acpi_osc_support(), so we no longer need to do it in the PCI
MSI driver.  Also adds the function pci_msi_enabled, which returns true
if pci=nomsi is not on the kernel command-line.
Signed-off-by: NAndrew Patterson <andrew.patterson@hp.com>
Signed-off-by: NJesse Barnes <jbarnes@virtuousgeek.org>