mac80211: Add IEEE80211_KEY_FLAG_PAIRWISE

This adds a new flag to the ieee80211_key_conf structure.
This flag will inform the driver the key is pairwise rather then
a shared key.

This is important for drivers who support both types of keys,
and need to be informed which type of key this is. Alternative
would be drivers checking the address argument of set_key(),
but it will be safer when mac80211 is more explicit.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>