net: dsa: don't use bitmap_weight() in b53_arl_read()

Don't call bitmap_weight() if the following code can get by
without it.
Signed-off-by: Yury Norov <yury.norov@gmail.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>

drivers/net/dsa/b53/b53_common.c

@@ -1624,12 +1624,8 @@ static int b53_arl_read(struct b53_device *dev, u64 mac,
 		return 0;
 	}
 
-	if (bitmap_weight(free_bins, dev->num_arl_bins) == 0)
-		return -ENOSPC;
-
 	*idx = find_first_bit(free_bins, dev->num_arl_bins);
-
-	return -ENOENT;
+	return *idx >= dev->num_arl_bins ? -ENOSPC : -ENOENT;
 }
 
 static int b53_arl_op(struct b53_device *dev, int op, int port,