iwlegacy: allow to enable PS

Power save support was removed from iwlegacy due to possible firmware
crashes problems it cause. I use to plan first inspect code to find
reason of problems, fix them and then allow to enable PS. But
realistically - code inspection will not happen, so let's do it, and
wait for eventual bug reports.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>