staging: vt6656: Fix drivers TBTT timing counter.

commit 09057742 upstream.

The drivers TBTT counter is not synchronized with mac80211 timestamp.

Reorder the functions and use vnt_update_next_tbtt to do the final
synchronize.

Fixes: c1515879

 ("staging: vt6656: implement TSF counter")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/375d0b25-e8bc-c8f7-9b10-6cc705d486ee@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>