ppc4xx_edac: Fix build error caused by wrong member access

Fix the following error

drivers/edac/ppc4xx_edac.c:977:45: error: request for member 'dimm' in something
not a structure or union

by changing member access to pointer dereference.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
Link: http://lkml.kernel.org/r/1408482646-22541-1-git-send-email-bobby.prani@gmail.com
CC: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Borislav Petkov <bp@suse.de>