mac80211: fix association with some APs

Some APs refuse association if the supported rates contained in the
association request do not match its own supported rates. This patch
introduces a new function which builds the intersection between the AP's
supported rates and the client's supported rates to work around such
problems. The same approach is already used in ipw2200 for example.

Signed-off-by: Helmut Schaa <hschaa@suse.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>