ar9170: xmit code revamp

This patch is a back-port from aggregation testing code.

In the past, we didn't limit the amount of active tx urbs.
However, ar9170 only has a limited buffer reserved for
pending data frames.

This wasn't much of a problem with the slower 802.11b/g.
We simply stopped the full queue and moved on to something different
in the mean time. But - as you guessed it -  this simple approach
stands in way for a decent aggregation implementation.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>