S2io: Check for device state before handling traffic

- Added check to return from the traffic handling function, if the card status
  is DOWN.
- Implemented Jeff's comments on incorrect return value in s2io_poll function.

Signed-off-by: Sivakumar Subramani <sivakumar.subramani@neterion.com>
Signed-off-by: Santosh Rastapur <santosh.rastapur@neterion.com>
Signed-off-by: Ramkrishna Vepa <ram.vepa@neterion.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>