rt2x00: fix regression introduced by "mac80211: free up 2 bytes in skb->cb"

The hw_key pointer is used (and obviously NULL) after skb->cb is
memset to 0. This patch grabs the iv_len before the memset call.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: Stephen Blackheath <tramp.enshrine.stephen@blacksapphire.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>