staging/fwserial: Destruct embedded tty_port on teardown

For TTY drivers that manage the port lifetime, the tty_port should
to be specifically destructed when the port lifetime ends. Now that
a method has been added to do this, use it.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>