libertas: fix buffer overrun

If somebody sends an invalid beacon/probe response, that can trash the
whole BSS descriptor. The descriptor is, luckily, large enough so that
it cannot scribble past the end of it; it's well above 400 bytes long.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Cc: stable@kernel.org [2.6.24-2.6.27, bug present in some form since driver was added (2.6.22)]
Signed-off-by: John W. Linville <linville@tuxdriver.com>