x86/mce: Update MCE severity condition check

Update some SRAR severity conditions check to make it clearer,
according to latest Intel SDM Vol 3(June 2013), table 15-20.

Signed-off-by: Chen Gong <gong.chen@linux.intel.com>
Acked-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Tony Luck <tony.luck@intel.com>