EDAC: Balance workqueue setup and teardown

We use the ->edac_check function pointers to determine whether we need
to setup a polling workqueue. However, the destroy path is not balanced
and we might try to teardown an unitialized workqueue.

Balance init and destroy paths by looking at ->edac_check in both cases.
Set op_state to OP_OFFLINE *before* destroying anything.

Reported-by: Zhiqiang Hou <Zhiqiang.Hou@freescale.com>
Cc: Varun Sethi <Varun.Sethi@freescale.com>
Signed-off-by: Borislav Petkov <bp@suse.de>