staging: vt6656: Power save stop wake_up_count wrap around.

commit ea81c348 upstream.

conf.listen_interval can sometimes be zero causing wake_up_count
to wrap around up to many beacons too late causing
CTRL-EVENT-BEACON-LOSS as in.

wpa_supplicant[795]: message repeated 45 times: [..CTRL-EVENT-BEACON-LOSS ]

Fixes: 43c93d9b

 ("staging: vt6656: implement power saving code.")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Link: https://lore.kernel.org/r/fce47bb5-7ca6-7671-5094-5c6107302f2b@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>