mac80211: Let drivers have access to TKIP key offets for TX and RX MIC

Some drivers may want to to use the TKIP key offsets for TX and RX
MIC so lets move this out. Lets also clear up a bit how this is used
internally in mac80211.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>