libertas: fix sleep confirmation

This fixes an issus that made "iwconfig eth1 power on" non-working.
When we get a "PS sleep" event, we have to confirm this to the firmware.
The confirm happens with a command, but this command is special: the
firmware won't send us a response. if_cs_host_to_card() is setting
priv->dnld_sent anyway, so this variable stayed at DNLD_DATA_SENT and
was never cleared back.

Now I put the special knowledge that the CMD_802_11_PS_MODE with
CMD_SUBCMD_SLEEP_CONFIRMED doesn't need to need a response by directly
clearing the dnld_sent state in lbs_send_confirmsleep().

Signed-off-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>