carl9170: add support for the new rate control API

With the new rate control API, the driver can now apply the
tx rate to outgoing frames just before they are uploaded to
the device. This is important because the rate control can
now react to fading or improving links a bit sooner.

Also, the driver no longer needs to sort the outgoing frames
for sample attempts (which affected the size of A-MPDUs and
the throughput of the link). For aggregated data frames, the
driver (and rate control) needs only to calculate and apply
a single set of tx rates to every subframe of the whole
aggregate.

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