rc80211-pid: add rate behaviour learning algorithm

This patch introduces a learning algorithm in order for the PID controller
to learn how to map adjustment values to rates. This is better described in
code comments.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>