mac80211: Shrink the size of ack_frame_id to make room for tx_time_est

To implement airtime queue limiting, we need to keep a running account of
the estimated airtime of all skbs queued into the device. Do to this
correctly, we need to store the airtime estimate into the skb so we can
decrease the outstanding balance when the skb is freed. This means that the
time estimate must be stored somewhere that will survive for the lifetime
of the skb.

To get this, decrease the size of the ack_frame_id field to 6 bits, and
lower the size of the ID space accordingly. This leaves 10 bits for use for
tx_time_est, which is enough to store a maximum of 4096 us, if we shift the
values so they become units of 4us.

Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
Link: https://lore.kernel.org/r/157182474063.150713.16132669599100802716.stgit@toke.dk
Signed-off-by: Johannes Berg <johannes.berg@intel.com>