[pci] Add PCI_COMMAND_INTX_DISABLE

Taken from Linux /usr/include/linux/pci.h .

Signed-off-by: Glenn Brown <glenn@myri.com>
Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Marty Connor <mdc@etherboot.org>

src/include/gpxe/pci.h

 #define  PCI_COMMAND_WAIT 	0x80	/* Enable address/data stepping */
 #define  PCI_COMMAND_SERR	0x100	/* Enable SERR */
 #define  PCI_COMMAND_FAST_BACK	0x200	/* Enable back-to-back writes */
+#define  PCI_COMMAND_INTX_DISABLE 0x400 /* INTx Emulation Disable */
 
 
 #define PCI_VENDOR_ID           0x00    /* 16 bits */