[PATCH] sis900: come alive after temporary memory shortage

[PATCH] sis900: come alive after temporary memory shortage

Patch solves following problems:
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 avaliable.
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>