bluetooth: delete timer in l2cap_conn_del()

Delete a possibly armed timer before kfree'ing the connection object.

Solves: http://lkml.org/lkml/2008/2/15/514

Reported-by: Quel Qun <kelk1@comcast.net>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>