mkiss: remove redundant assignment of len to ax->mtu

Variable len is being assigned a value that is never read,
hence the assignment is redundant and can be removed. Cleans
up clang warning:

drivers/net/hamradio/mkiss.c:443:3: warning: Value stored to
'len' is never read
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/hamradio/mkiss.c

@@ -440,7 +440,6 @@ static void ax_encaps(struct net_device *dev, unsigned char *icp, int len)
 		ax_changedmtu(ax);
 
 	if (len > ax->mtu) {		/* Sigh, shouldn't occur BUT ... */
-		len = ax->mtu;
 		printk(KERN_ERR "mkiss: %s: truncating oversized transmit packet!\n", ax->dev->name);
 		dev->stats.tx_dropped++;
 		netif_start_queue(dev);