* include opt_wlan.h like a good little wlan driver;
* add a function to free the mbufq /and/ the node references on it, or we will leak
said node references;
* free the mbufq upon NIC shutdown otherwise we may end up with a full list that
we never begin transmit work on, and thus never drain it;
* .. which frees it upon NIC detach too;
* ensure urtwn_start() gets called after the completion of frame TX even if the
pending queue is empty, otherwise transmit will stall. It's highly unlikely that
the usb tx queue would be empty whilst the incoming send queue is full, but hey,
who knows.
This passes some iperf testing with and without the NIC being actively removed during
said active iperf test.
Tested:
* urtwn0: MAC/BB RTL8188EU, RF 6052 1T1R ; STA mode
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