EDAC: Correct workqueue setup path

We're using edac_mc_workq_setup() both on the init path, when
we load an edac driver and when we change the polling period
(edac_mc_reset_delay_period) through /sys/.../edac_mc_poll_msec.

On that second path we don't need to init the workqueue which has been
initialized already.

Thanks to Tejun for workqueue insights.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1391457913-881-1-git-send-email-prarit@redhat.com
Cc: <stable@vger.kernel.org>