Bluetooth: Clear RFCOMM session timer when disconnecting last channel

When the last RFCOMM data channel is closed, a timer is normally set
up to disconnect the control channel at a later time.  If the control
channel disconnect command is sent with the timer pending, the timer
needs to be cancelled.

If the timer is not cancelled in this situation, the reference
counting logic for the RFCOMM session does not work correctly when the
remote device closes the L2CAP connection.  The session is freed at
the wrong time, leading to a kernel panic.

CRs-fixed: 317859
Change-Id: I778fdf8dbff8afe0a1c157b02d39872262088584
Signed-off-by: Mat Martineau <mathewm@codeaurora.org>