tty/8250 Add XR17D15x devices to the exar_handle_irq override

Add XR17D15x devices to the exar_handle_irq override: they have the
same extra interrupt register that could fire and never be serviced by
the standard handle_irq.

Signed-off-by: Matt Schulte <matts@commtech-fastcom.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>