mac80211: Replace ieee80211_tx_control->key_idx with ieee80211_key_conf

The hw_key_idx inside the ieee80211_key_conf structure does
not provide all the information drivers might need to perform
hardware encryption.

This is in particular true for rt2x00 who needs to know the
key algorithm and whether it is a shared or pairwise key.

By passing the ieee80211_key_conf pointer it assures us that
drivers can make full use of all information that it should know
about a particular key.

Additionally this patch updates all drivers to grab the hw_key_idx from
the ieee80211_key_conf structure.

v2: Removed bogus u16 cast
v3: Add warning about ieee80211_tx_control pointers
v4: Update warning about ieee80211_tx_control pointers

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>