ipw2x00: change default policy for auto-associate

Do not attempt association until directed to do so by a user space
application. In particular, this avoids race conditions with
NetworkManager association state.

Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>