edac: sb_edac: Fix a wrong value setting for the previous value

>From the driver design, the variable limit wants to compare with its
previous value, we should set the value of limit instead of the value
of tmp_mb to the variable prev.

Signed-off-by: Hui Wang <jason77.wang@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>