iwlegacy: warn when enabling power save

iwlegacy firmware can crash when power save is configured. PS was
allowed in "dbdac2b5 iwlegacy: properly enable power saving" with belive
that user who enable PS is aware of that and can relate firmware crahes
with PS. However some distributions seems to enable PS without user
intervention, so warn about that.

Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>