rc80211-pid: pf_target tuning

Set a better value for percentage target for failed frames. The previous value
slowed down too much rate increases in case of permanently low activity. While
at it, increase readability.

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>