libertas: kill internal tx queue for PS mode

It was buggy as hell anyway, since it was just spewing packets at the
device when it wasn't necessarily ready for them (in the USB case, while
the URB was still busy).

We could probably do with a better way of flushing packets to the device
_immediately_, before we stick it back into sleep mode. But we can no
longer just dequeue packets directly, it seems.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>