Bluetooth: Proper shutdown ERTM when closing the channel

Fix a crash regarding the Monitor Timeout, it was running even after the
shutdown of the ACL connection, which doesn't make sense.

The same code also fixes another issue, before this patch L2CAP was sending
many Disconnections Requests while we have to send only one.

The issues are related to each other, a expired Monitor Timeout can
trigger a Disconnection Request and then we may have a crash if the link
was already deleted.

Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>