ath9k: fixing register bit shift values of control packets to support TPC

Some register values of bit shifts are corrected in order to support the upcoming
transmission power control (tpc) for control packets as well.

Signed-off-by: Thomas Huehn <thomas@net.t-labs.tu-berlin.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>