ipw2200: make association only if SSID is known.

'iwconfig eth1 channel 6' would trigger association to _something_,
which is wrong.  Changing the channel should (and does) trigger reassociation,
but only if there is an SSID to associate with.

Signed-off-by: Alexey Fisher <bug-track@fisher-privat.net>
Acked-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>