serial_core: Fix race in uart_handle_dcd_change

If a serial driver is called post hangup with a second DCD event then we
will attempt to get the ldisc of NULL. Check we have a tty before trying to
do anything with it.

This is still only safe within the uart layer if the caller holds the
relevant uart locks. We could do with a version where the tty is passed for
more general use.

Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>