sb_edac: make minimal use of channel_mask

channel_mask will be used in the future to determine which group of memory
modules is causing the errors since when mirroring, lockstep and close page
are enabled you can't. While that doesn't happen, use the channel_mask to
determine the channel instead of relying on the MC event/exception.

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>