iwlagn: add comment to warn about WoWLAN in resume / suspend flows

WoWLAN may need the NIC even after suspend. One should not do anything to the
NIC in the bus level, since one cannot check whether WoWLAN is enabled or not.
Same for resume.

Add a simple comment to the code to warn about this.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>