mac80211: get a TKIP phase key from skb

This patch makes mac80211 able to compute a TKIP key from an skb.
The requested key can be a phase 1 or a phase 2 key.
This is useful for drivers who need to provide tkip key to their
HW to enable HW encryption.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>