6pack: drop sixpack::mtu

It holds a constant (AX25_MTU + 73), so use that constant in place of
the single use directly.

And remove the stale comment.

Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Andreas Koensgen <ajk@comnets.uni-bremen.de>
Cc: David S. Miller <davem@davemloft.net>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Paolo Abeni <pabeni@redhat.com>
Cc: linux-hams@vger.kernel.org
Cc: netdev@vger.kernel.org
Reviewed-by: Jeremy Kerr <jk@codeconstruct.com.au>
Link: https://lore.kernel.org/r/20240808103549.429349-9-jirislaby@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: Jiri Slaby (SUSE)

drivers/net/hamradio/6pack.c

@@ -100,7 +100,6 @@ struct sixpack {
 	unsigned int		rx_count_cooked;
 	spinlock_t		rxlock;
 
-	int			mtu;		/* Our mtu (to spot changes!) */
 	int			buffsize;       /* Max buffers sizes */
 
 	unsigned long		flags;		/* Flag values/ mode etc */
@@ -166,7 +165,7 @@ static void sp_encaps(struct sixpack *sp, unsigned char *icp, int len)
 	unsigned char *msg, *p = icp;
 	int actual, count;
 
-	if (len > sp->mtu) {	/* sp->mtu = AX25_MTU = max. PACLEN = 256 */
+	if (len > AX25_MTU + 73) {
 		msg = "oversized transmit packet!";
 		goto out_drop;
 	}
@@ -585,7 +584,6 @@ static int sixpack_open(struct tty_struct *tty)
 
 	sp->xbuff	= xbuff;
 
-	sp->mtu		= AX25_MTU + 73;
 	sp->buffsize	= len;
 	sp->rcount	= 0;
 	sp->rx_count	= 0;