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>