iwlegacy: properly enable power saving

Even if we mark PS on, device still worked in normal mode. Patch
corrects that and now we send proper powertable command to device,
which put it in sleep mode when PS is on.

Reported-and-tested-by: Tino Keitel <tino.keitel@tikei.de>
Tested-by: Pedro Francisco <pedrogfrancisco@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>