agp/ati: Use PCI_COMMAND instead of hard-coded 4

We're accessing the PCI_COMMAND register here, so use the appropriate
#define.  The bit we're writing (1 << 14) isn't defined by the PCI or PCIe
spec, so we don't have a name for it.

No functional change.

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