niu: Fix missing unwind goto in niu_alloc_channels()

Smatch reports: drivers/net/ethernet/sun/niu.c:4525
	niu_alloc_channels() warn: missing unwind goto?

If niu_rbr_fill() fails, then we are directly returning 'err' without
freeing the channels.

Fix this by changing direct return to a goto 'out_err'.

Fixes: a3138df9f2 ("[NIU]: Add Sun Neptune ethernet driver.")
Signed-off-by: Harshit Mogalapalli <harshit.m.mogalapalli@oracle.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/sun/niu.c

@@ -4522,7 +4522,7 @@ static int niu_alloc_channels(struct niu *np)
 
 		err = niu_rbr_fill(np, rp, GFP_KERNEL);
 		if (err)
-			return err;
+			goto out_err;
 	}
 
 	tx_rings = kcalloc(num_tx_rings, sizeof(struct tx_ring_info),