iwlegacy: make iwl3945 and iwl4965 select IWLWIFI_LEGACY

Otherwise, IWLWIFI_LEGACY has to be selected independently before the
drivers are made available.

Reported-by: Dave Airlie <airlied@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
Cc: Stanislaw Gruszka <sgruszka@redhat.com>
Cc: Wey-Yi Guy <wey-yi.w.guy@intel.com>