EDAC: Rework workqueue handling

Hide the EDAC workqueue pointer in a separate compilation unit and add
accessors for the workqueue manipulations needed.

Remove edac_pci_reset_delay_period() which wasn't used by anything. It
seems it got added without a user with

  91b99041 ("drivers/edac: updated PCI monitoring")

Signed-off-by: Borislav Petkov <bp@suse.de>