libertas: fix memory alignment problems on the blackfin

Fixing unaligned memory access on the blackfin architecture (maybe on the
ARM also).

Signed-off-by: Ihar Hrachyshka <ihar.hrachyshka@promwad.com>
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>
Signed-off-by: David S. Miller <davem@davemloft.net>