NET: AX.25: Stop heartbeat timer on disconnect.

This may result in a kernel panic.  The bug has always existed but
somehow we've run out of luck now and it bites.

Signed-off-by: Richard Stearn <richard@rns-stearn.demon.co.uk>
Cc: stable@vger.kernel.org	# all branches
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
Signed-off-by: David S. Miller <davem@davemloft.net>