Bluetooth: Purge the dlc->tx_queue to avoid circular dependency

In rfcomm_tty_cleanup we purge the dlc->tx_queue which may contain
socket buffers referencing the tty_port and thus preventing the tty_port
destruction.

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>