sis900: come alive after temporary memory shortage

1) Forgotten counter incrementation in sis900_rx() in case
     it doesn't get memory for skb, that leads to whole interface failure.
     Problem is accompanied with messages:
    eth0: Memory squeeze,deferring packet.
    eth0: NULL pointer encountered in Rx ring, skipping

2) If counter cur_rx overflows and there'll be temporary memory problems
     buffer can't be recreated later, when memory IS available.

3) Limit the work in handler to prevent the endless packets processing
   if new packets are generated faster then handled.

Signed-off-by: Konstantin Khorenko <khorenko@sw.ru>
Signed-off-by: Vasily Averin <vvs@sw.ru>
Signed-off-by: Jeff Garzik <jgarzik@pobox.com>