libertas: convert PS_MODE to a direct command

Powersave looks like it got broken at some point but we'll fix that up
when the command submission stuff is more understandable, which this
series helps to do.  That said, this patch should not further break
powersave.

Signed-off-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>