iwlwifi: support idle for 6000 series hw

Using powersave while idle saves a lot of power, but
we've had problems with this on some cards (5150 has
been reported to be problematic). However, on the new
6000 series we're seeing no problems, so for now let
that hardware benefit from idle mode, we can look at
the problems with other hardware one by one and then
enable those once we figure out the problems.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>