net: macb: remove redundant assignment to variable work_done

Variable work_done is set to zero and this value is never read, instead
it is set to another value a few statements later.  Remove the redundant
assignment. Cleans up clang warning:

drivers/net/ethernet/cadence/macb_main.c:1221:2: warning: Value stored
to 'work_done' is never read
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Tested-by: NAlexander Dahl <ada@thorsis.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/macb_main.c

@@ -1218,8 +1218,6 @@ static int macb_poll(struct napi_struct *napi, int budget)
 	status = macb_readl(bp, RSR);
 	macb_writel(bp, RSR, status);
 
-	work_done = 0;
-
 	netdev_vdbg(bp->dev, "poll: status = %08lx, budget = %d\n",
 		    (unsigned long)status, budget);