staging: vt6656: implement TSF counter

The TSF counter is presently unused on driver in infrastructure mode.

It is needed to implement power saving functions and wireless stats that
are passed in interrupt handling.

Ensuring counter is synced with access point beacon.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>