libertas: un-garbage various command structs

Some command structs contain reserved or unused fields, which the driver
send uninitialized down to the card.

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>