[IA64-SGI] fix sn_pci_legacy_read/fix sn_pci_legacy_write

This patch adds a #define for SN_SAL_IOIF_PCI_SAFE and makes that the
preferred method of implementing sn_pci_legacy_read() and
sn_pci_legacy_write().

This SAL call has been present in SGI proms since version 4.10.  If the
SN_SAL_IOIF_PCI_SAFE call fails, revert to the previous code for compatability
with older proms.

Signed-off-by: Mark Maule <maule@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>