iwlagn: check for SMPS mode

commit b2ccccdc upstream.

Check and report WARN only when its invalid

Resolves:
https://bugzilla.kernel.org/show_bug.cgi?id=42621
https://bugzilla.redhat.com/show_bug.cgi?id=766071



Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>