mac80211: fix basic rate bitmap calculation

"mac80211: fix basic rates setting from association response"
introduced a copy/paste error.

Unfortunately, this not just leads to wrong data being passed
to the driver but is remotely exploitable for some hardware or
driver combinations.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.29]
Signed-off-by: John W. Linville <linville@tuxdriver.com>