libertas: Read outside array bounds

reads bss->rates[j] before checking bounds of index, and should use
ARRAY_SIZE to determine the size of the array.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Acked-by: Holger Schurig <hs4233@mail.mn-solutions.de>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>