iwlwifi: refactor some thermal throttle code

Some of the thermal throttle data structures and code
are really very intermingled with the sleep (power)
control code. They really do belong together in a way
since the thermal throttle code uses powersaving to
achieve its goal, but it's making it hard to work on
the powersave code. Split this up to make that easier.
I've also changed the antenna defines to an enum and
used the same enum for RX and TX.

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>