ath5k: enable combined michael mic in key cache

For mac revisions >= "Griffin," the hardware allows the mic tx and rx
authenticator keys to share the same cache line, whereas earlier
hardware can only store the rx.  Enable the combined mic on hardware
that supports it.

Changes to ath5k.h
Changes-licensed-under: 3-Clause-BSD

Changes to attach.c, pcu.c, reg.h
Changes-licensed-under: ISC

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>