rc80211-pid: add sharpening factor

This patch introduces a PID sharpening factor for faster response after
association and low activity events.

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