drivers/edac: fix workq reset deadlock

Fix mutex locking deadlock on the device controller linked list.  Was calling
a lock then a function that could call the same lock.  Moved the cancel workq
function to outside the lock

Added some short circuit logic in the workq code

Added comments of description

Code tidying

Signed-off-by: Doug Thompson <dougthompson@xmission.com>
Cc: Greg KH <greg@kroah.com>
Cc: Alan Cox <alan@lxorguk.ukuu.org.uk>
Cc: Oleg Nesterov <oleg@tv-sign.ru>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>