sb_edac: fix socket detection on Ivy Bridge controllers

This patch fixes the obvious bug while handling the socket/HA bitmask used in
Ivy Bridge memory controllers.

Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>