mac80211: provide sequence numbers

I've come to think that not providing sequence numbers for
the normal STA mode case was a mistake, at least two drivers
now had to implement code they wouldn't otherwise need, and
I believe at76_usb and adm8211 might be broken.

This patch makes mac80211 assign a sequence number to all
those frames that need one except beacons. That means that
if a driver only implements modes that do not do beaconing
it need not worry about the sequence number.

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

net/mac80211/ieee80211_i.h

@@ -438,6 +438,8 @@ struct ieee80211_sub_if_data {
 	struct ieee80211_key *keys[NUM_DEFAULT_KEYS];
 	struct ieee80211_key *default_key;
 
+	u16 sequence_number;
+
 	/* BSS configuration for this interface. */
 	struct ieee80211_bss_conf bss_conf;