iwlagn: invoke L0S workaround for 6000/1000 series

Invoke workaround to avoid instability in L0->L0S->L1 transition on PCIe bus.
Workaround disables L0S state so device moves directly from L0->L1.
Workaround needed on all devices since and including 4965; add to 6000/1000.

Describe bug and workaround better in comments.

Signed-off-by: Ben Cahill <ben.m.cahill@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>