p54: Fix for TX sequence number problem

Following "mac80211: fix TX sequence numbers", if a packet
has the IEEE80211_TX_CTL_ASSIGN_SEQ assigned, a sequence number must be
supplied, either by hardware or software. AFAIK, no such hardware exists
for the p54, thus it must be done in software. With this patch, a connection
qith p54usb is stable, whereas the interface went off-line in 2-3 hours
without this change. Note that this code will have to be reworked for proper
sequence numbers on beacons. In addition, the sequence number has been placed
in the hardware state, not the vif state.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>