staging: rtl8723au: Move dummy_event_callback() to rtw_mlme_ext.c

In addition get rid of a couple of empty *_event_callback() handlers
and use dummy_event_callback() instead.

Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>