sb_edac: avoid decoding the same error multiple times

Whenever the extended error reporting is active, multiple MCEs will be
generated for the same event, which will lead to multiple repeated
errors to be reported. So check ADDRV and only decode the error if the
MCE address is valid.

Signed-off-by: Aristeu Rozanski <arozansk@redhat.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>