amd64_edac: fix driver instance freeing

Fix use-after-free errors by pushing all memory-freeing calls to the end
of amd64_remove_one_instance().

Reported-by: Darren Jenkins <darrenrjenkins@gmail.com>
LKML-Reference: <1261370306.11354.52.camel@ICE-BOX>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>