EDAC, mv64x60_edac: Fix an error code in probe()

If edac_mc_add_mc() fails then we should preserve the error code, but
instead the current code returns success.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: http://lkml.kernel.org/r/20150128191351.GC10259@mwanda


Signed-off-by: Borislav Petkov <bp@suse.de>