staging: rtl8723au: clean up update_TSF()

Clean up and simplify update_TSF() using proper Linux functions and
move it to rtw_mlme_ext.c which is the only user of it.

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

diff --cc drivers/staging/rtl8723au/core/rtw_mlme_ext.c
Simple merge

diff --cc drivers/staging/rtl8723au/core/rtw_wlan_util.c
Simple merge

diff --cc drivers/staging/rtl8723au/include/rtw_mlme_ext.h
Simple merge