sb_edac: Fix typo computing number of banks

Code will always think there are 16 banks because of a typo

Reported-by: Misha
Signed-off-by: Tony Luck <tony.luck@intel.com>
Acked-by: Aristeu Rozanski <aris@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>