mac80211: move tx crypto decision

This patch moves the decision making about whether a frame is encrypted
with a certain algorithm up into the TX handlers rather than having it
in the crypto algorithm implementation.

This fixes a problem with the radiotap injection code where injecting
a non-data packet and requesting encryption could end up asking the
driver to encrypt a packet without giving it a key.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>