wlcore: save last regdom configuration on stop

In order to reconfigure the correct reg domain on
recovery, we have to save the current configuration
before clearing it (wl->reg_ch_conf_pending is
considered before configuring a new regdomain).

Signed-off-by: Eliad Peller <eliad@wizery.com>
Signed-off-by: Luciano Coelho <luciano.coelho@intel.com>