[IA64-SGI] fix prom revision checks in SN kernel

The following patch fixes two spots in the SN kernel
that check a fixed prom revision number to determine prom
feature support. These checks are only valid on shub1 systems.
They are invalid on shub2 systems which have a different prom
with different revision numbers.

Signed-off-by: Aaron Young <ayoung@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>