[IA64] mbcs_init() should give up unless running on sn2

CONFIG_SGI_MBCS is enabled in generic kernels, but the driver may
oops some other platforms.  Check whether we are running on sn2
and bail out if we are not before doing anything dangerous.

Acked-by: Bruce Losure <blosure@americas.sgi.com>
Signed-off-by: Greg Edwards <edwardsg@sgi.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>