iwlwifi: fix dynamic loading

Add locking to the dynamic loading code to prevent
corrupting the list if multiple device ever init at
the same time (which cannot happen for multiple PCI
devices, but could happen when different busses init
concurrently.)

Also remove a device from the list when it stops so
the list isn't left corrupted, including a fix from
Don to not crash when it was never added.

Reviewed-by: Donald H Fry <donald.h.fry@intel.com>
Tested-by: Donald H Fry <donald.h.fry@intel.com>
Reviewed-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Don Fry <donald.h.fry@intel.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>