b43: Rewrite TX power adjustment

This patch rewrites the TX power recalculation algorithms to scale better
with changed enviromnent. If there's low
TX traffic, the power will be checked against the desired values
every 60 seconds.
If there is high TX traffic, the check is redone every 2 seconds. This improves
the reaction times a lot and confuses the rate control less.
It will also reduce the time it initially takes to tune to a new TX power
value. With the old algorithm it could take about 30 to 45 seconds to settle to
a new power value. This will happen in about two to four seconds now.

Signed-off-by: Michael Buesch <mb@bu3sch.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>