ieee1394: nodemgr: grab class.subsys.rwsem in nodemgr_resume_ne

nodemgr_resume_ne was iterating over nodemgr_ud_class.children without
protection by nodemgr_ud_class.subsys.rwsem.

FIXME:
Shouldn't we rather use class->sem there, not class->subsys.rwsem?

Signed-off-by: Stefan Richter <stefanr@s5r6.in-berlin.de>