amd64_edac: Fix driver module removal

f4347553 removed the edac polling
mechanism in favor of using a notifier chain for conveying MCE
information to edac. However, the module removal path didn't test
whether the driver had setup the polling function workqueue at all and
the rmmod process was hanging in the kernel at try_to_del_timer_sync()
in the cancel_delayed_work() path, trying to cancel an uninitialized
work struct.

Fix that by adding a balancing check to the workqueue removal path.

Signed-off-by: Borislav Petkov <borislav.petkov@amd.com>