USB: serial: call handle_dcd_change in ftdi driver.

When the device receive a DCD status change, forward the signal to the
USB serial system. This way, we can detect, for instance, PPS pulses.

Signed-off-by: Paul Chavent <paul.chavent@onera.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>