[SUNGEM]: Fix NAPI assertion failure.

As reported by Johannes Berg:

I started getting this warning with recent kernels:

[  773.908927] ------------[ cut here ]------------
[  773.908954] Badness at net/core/dev.c:2204
 ...

If we loop more than once in gem_poll(), we'll
use more than the real budget in our gem_rx()
calls, thus eventually trigger the caller's
assertions in net_rx_action().

Subtract "work_done" from "budget" for the second
arg to gem_rx() to fix the bug.
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/sungem.c

@@ -912,7 +912,7 @@ static int gem_poll(struct napi_struct *napi, int budget)
 		 * rx ring - must call napi_disable(), which
 		 * schedule_timeout()'s if polling is already disabled.
 		 */
-		work_done += gem_rx(gp, budget);
+		work_done += gem_rx(gp, budget - work_done);
 
 		if (work_done >= budget)
 			return work_done;