ath5k: update keycache to support TKIP handling

Newer parts have slots at entry+64 for michael mic and can do WPA-TKIP
in hardware.  The open-sourced Atheros HAL has code for accessing this
portion so now we know how where to put the key material.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Acked-by: Nick Kossifidis <mickflemm@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>