EDAC: Fix workqueue-related crashes

EDAC: Fix workqueue-related crashes

00740c58541b6087d78418cebca1fcb86dc6077d changed edac_core to
un-/register a workqueue item only if a lowlevel driver supplies a
polling routine. Normally, when we remove a polling low-level driver, we
go and cancel all the queued work. However, the workqueue unreg happens
based on the ->op_state setting, and edac_mc_del_mc() sets this to
OP_OFFLINE _before_ we cancel the work item, leading to NULL ptr oops on
the workqueue list.

Fix it by putting the unreg stuff in proper order.

Cc: <stable@kernel.org> #36.x
Reported-and-tested-by: Tobias Karnat <tobias.karnat@googlemail.com>
LKML-Reference: <1291201307.3029.21.camel@Tobias-Karnat>
Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>