libertas: don't clear priv->dnld_sent after sending sleep confirm

DNLD_RES_RECEIVED is a bit of a misnomer -- we never wait for the result
to be received; it's purely representing the state of the TX path, and
in this case the TX path is definitely busy.

Of course, that means that we don't actually care about DATA_SENT vs.
CMD_SENT either, but that's a can of worms for another day...

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