ath5k: Use common ath key management functions

Use common ath key management functions in ath5k. This fixes problems with HW
encryption in AP mode, which was broken in the ath5k implementation.

Before (with the ath5k implementation) only one client could connect to the AP
using HW encryption and WPA. When a second client connected, the first client
was not able to send/receive any more packets. Because of the problems with HW
encryption, software encryption was always used in AP mode, which resulted in a
high CPU load (and/or low thruput) on embedded devices. Instead of trying to
fix the implementation in ath5k it makes more sense to share the code with
ath9k.

This also enables HW encryption for AP mode again.

Signed-off-by: Bruno Randolf <br1@einfach.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>