mac80211: set TX_CTL_ASSIGN_SEQ for group QoS frames

According to 802.11-2012 9.3.2.10, paragraph 4, QoS
data frames with group addresses in the Address 1 field
have sequence numbers allocated from the same counter
as non-QoS data and management frames.  Without this
flag, some drivers may not assign sequence numbers,
causing some frames to get dropped.  Set the control
flag accordingly.

Signed-off-by: Bob Copeland <me@bobcopeland.com>

---
I noticed seq=0 on DHCP/mDNS frames with mesh (since mesh
sends all data frames as QoS).  It seems Atheros kit will
already do the right thing without the flag, but other
hardware needed it.
mwl8787: support tx_ctl_assign_seq

This doesn't really work because beacons don't know about the counter,
but it's slightly better than setting all zeroes in the sequence
field.  Firmware should either assign the sequence numbers by looking
at the frames or by passing this flag in the tx descriptor control
or flags fields.

Signed-off-by: Bob Copeland <me@bobcopeland.com>