staging: rtl8723au: update_TSF() remove unused argument and move to rtw_mlme_ext.c

This is only called from rtw_mlme_ext.c, so move it that and declare
it static.

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