iwlwifi: use sleep interval succession

Some concerns were raised about the automatic adjustment
of sleep intervals to all the same, potentially high,
value, and I can imagine the hardware behaving better
when we don't ask too much of it.

So let's convert to use a succession of sleep levels
when requesting to go to deeper sleeps (which can only
happen with large DTIM intervals), using the succession
values from power level three, which have the benefit of
also having been tested extensively already.

As a result, the automatic sleep level adjustment will
now be mostly equivalent to power level three, except
for the RX/TX timeouts and possibly using smaller sleep
vectors to account for networking latency.

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>