rc80211-pid: fix rate adjustment

Merge rate_control_pid_shift_adjust() to rate_control_pid_adjust_rate()
in order to make the learning algorithm aware of constraints on rates. Also
add some comments and rename variables.

This fixes a bug which prevented 802.11b/g non-AP STAs from working with
802.11b only AP STAs.

Signed-off-by: Stefano Brivio <stefano.brivio@polimi.it>
Signed-off-by: John W. Linville <linville@tuxdriver.com>