x86, uv, uv3: Check current gru hub support for SGI UV3

This patch checks current hub support to avoid panicing the
system until all the GRU changes for UV3+ are in place.

Signed-off-by: Mike Travis <travis@sgi.com>
Link: http://lkml.kernel.org/r/20130211194509.035828372@gulag1.americas.sgi.com
Acked-by: Dimitri Sivanich <sivanich@sgi.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>