sb_edac: Fix off-by-one error in number of channels

This prevented edac sysfs code from properly handling 6 channels
per memory controller.

Signed-off-by: Jim Snow <jim.snow@intel.com>
Signed-off-by: Lukasz Anaczkowski <lukasz.anaczkowski@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>