rt2x00: remove skb->do_not_encrypt usage

Johannes is trying to get rid of the master netdev and in the process will
remove skb->do_not_encrypt field. This removes the do_not_encrypt
usage from rt2x00 to make the change easier.

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