libertas: fix association with some APs by using extended rates

Some APs get pissy if you don't send the firmware the extended rates
in the association request's rates TLV.  Found this on a Linksys
WRT54G v2; it denies association with status code 18 unless you
add the extended rates too.  The old driver did this, but it got
lost in the cfg80211 conversion.

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