rc80211-pid: simplify and fix shift_adjust

Simplify and fix rate_control_pid_shift_adjust(). A bug prevented correct
mapping of sorted rates, and readability was seriously flawed.

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>