iwlwifi/Kconfg: make IWLWIFI_LEDS invisible

Select IWLWIFI_LEDS automatically when either IWL3945_LEDS or
IWL4965_LEDS is selected.  This avoids potential misconfigurations
which lead to build failures for iwl-led.c.

Cc: Miles Lane <miles.lane@gmail.com>
Cc: reinette.chatre@intel.com
Signed-off-by: John w. Linville <linville@tuxdriver.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>