edac: sb_edac: Fix a INTERLEAVE_MODE() misuse

We can identify dram interleave mode from the Dram Rule register
rather than Dram Interleave list register.

In this context, the reg of INTERLEAVE_MODE(reg) contains the Dram
Interleave list register, we can't get interleave mode from the reg,
while the variable interleave_mode saves the the mode got from the
Dram Rule register, so we use the variable to replace
INTERLEAVE_MDDE(reg) here.

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