pci: make MSI(-X) enable and disable methods of the PCI bus

Make the functions pci_disable_msi, pci_enable_msi and pci_enable_msix
methods of the newbus PCI bus. This code should not include any
functional change.

Sponsored by: Citrix Systems R&D
Reviewed by: imp, jhb
Differential Revision: https://reviews.freebsd.org/D354

dev/pci/pci.c:
 - Convert the mentioned functions to newbus methods.
 - Fix the callers of the converted functions.

sys/dev/pci/pci_private.h:
dev/pci/pci_if.m:
 - Declare the new methods.

dev/pci/pcivar.h:
 - Add helpers to call the newbus methods.

ofed/include/linux/pci.h:
 - Add define to prevent the ofed version of pci_enable_msix from
   clashing with the FreeBSD native version.
Add missing break.

CID:	603368