MSI: Factorize common code in pci_msi_supported()

pci_enable_msi() and pci_enable_msix() use the same code to detect
whether MSI might be enabled on this device. Factorize this code in
pci_msi_supported(). And improve the documentation about the fact
that only the root chipset must support MSI, but it is hard to
find the root bus so we check all parent busses MSI flags.

Signed-off-by: Brice Goglin <brice@myri.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>