Bluetooth: don't release the port in rfcomm_dev_state_change()

Bluetooth: don't release the port in rfcomm_dev_state_change()

When the dlc is closed, rfcomm_dev_state_change() tries to release the
port in the case it cannot get a reference to the tty. However this is
racy and not even needed.

Infact as Peter Hurley points out:

1. Only consider dlcs that are 'stolen' from a connected socket, ie.
   reused. Allocated dlcs cannot have been closed prior to port
   activate and so for these dlcs a tty reference will always be avail
   in rfcomm_dev_state_change() -- except for the conditions covered by
   #2b below.
2. If a tty was at some point previously created for this rfcomm, then
   either
   (a) the tty reference is still avail, so rfcomm_dev_state_change()
       will perform a hangup. So nothing to do, or,
   (b) the tty reference is no longer avail, and the tty_port will be
       destroyed by the last tty_port_put() in rfcomm_tty_cleanup.
       Again, no action required.
3. Prior to obtaining the dlc lock in rfcomm_dev_add(),
   rfcomm_dev_state_change() will not 'see' a rfcomm_dev so nothing to
   do here.
4. After releasing the dlc lock in rfcomm_dev_add(),
   rfcomm_dev_state_change() will 'see' an incomplete rfcomm_dev if a
   tty reference could not be obtained. Again, the best thing to do here
   is nothing. Any future attempted open() will block on
   rfcomm_dev_carrier_raised(). The unconnected device will exist until
   released by ioctl(RFCOMMRELEASEDEV).

The patch removes the aforementioned code and uses the
tty_port_tty_hangup() helper to hangup the tty.

Signed-off-by: Gianluca Anzolin <gianluca@sottospazio.it>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>