staging: vt6656: implement power saving code.

The device should ready to listen to next beacon so on count down of
wake_up_count == 1.

schedule command WLAN_CMD_TBTT_WAKEUP which calls vnt_next_tbtt_wakeup which
wakes the device.

This restores this back to vendors code with a slight difference the
count down is in the interrupt handler.

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