rt2x00: Fix QOS sequence counting

When IEEE80211_TX_CTL_ASSIGN_SEQ is not set,
the driver should disable hardware sequence counting
to make sure the mac80211 provided counter is used.
This fixes QOS sequence counting, since that is one
of the cases where mac80211 provides a seperate
sequence counter.

By moving the sequence counting code to rt2x00queue
we make sure that _all_ frames get the sequence counter,
including RTS/CTS and Beacon frames.

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